What are the reasons for the increased N:P ratio during the last decades in the rivers discharging to coastal waters:
(1) the ratio has increased due to removal of phosphate rich detergents.
(2) the ratio has declined because agriculture uses less mineral Nfertilizers.
(3) the ratio has increased because sewage treatments plants remove P more efficiently than N.
(4) the ratio has declined because the sewage treatment plants remove almost all the N.
(5) the ratio has not changed.
There may be more than one correct choice.
Based on the information provided, I'm assuming that this question is not referring to any specific data on coastal discharge or water treatment, but rather, it is more concerned with your understanding of ratios and the way that the question is stated. We can arrive at two possible answers by evaluating the information and language in the question itself.
Ratios are defined as a value of one thing in relation to the value of another. While the values remain the same, the order in which they are compared can change; this alters the language necessary to describe them.
The question suggests that the ratio of N to P has increased. This is to say that it is suggested that the value of N, in relation to P, has gone up. While N represents concentrations of nitrogen and P represents concentrations of phosphorous in river water, we could make many analogies with this, such as saying that N and P are the cost of a bag of chips. Logically, if you can buy N bags of chips with P dollars, and the ratio of N to P increases, that means you can buy more bags of chips with the same amount of money. There are two ways to satisfy this relationship; increase the value of N, or decrease the value of P.
If we accept the statement in the question as a statement of fact instead of as a suggestion to be examined and determined to be true or false based on river discharge analysis, we can examine the logical application of the possibilities presented for changing a ratio.
In context of logical application of ratio relationships, in evaluating the options given to us in the question, we can logically eliminate three. Option 5 is immediately eliminated, because the ratio cannot increase, yet remain the same. Option 2 is eliminated because it directly contradicts the question's conditions; decreasing N will not increase the ratio of N to P. Likewise, option 4 is eliminated because removing almost all the N will not increase the ratio.
Option 1 is viable because reducing the amount of P increases the number of N per P, thereby increasing the ratio.
Option 3 is viable because both are being reduced, but the rate of P reduction is greater. I should note that the language of this option ought to be revised in order to be a truly accurate option, because efficiency does not necessarily imply the amount of P being removed; it could be referring to the cost instead.
Directing attention away from logical operation of ratios to the actual topic of your question, knowing which river-coastal water discharge system you are studying is a prerequisite to knowing whether the nitrogen to phosphorous ratio has increased or declined and to what these ratio fluctuations may be due. We can offer two examples, though, one where the ratio has declined and one where the ratio has increased.
Studies conducted between 1960 and 2000 on the river-coastal discharge system for Mediterranean convergence show an increased N:P ratio. The cause of the increase is attributable to unspecified anthropogenic activity: "were strongly enhanced by anthropogenic sources."
the fluxes of nitrogen (N) and phosphorus (P) in Mediterranean ... rivers, which were strongly enhanced by anthropogenic sources ... [the] total inputs to the Mediterranean Sea could have increased by a factor of >5.
In this river-coastal water discharge system, while the ratio has increased, it is because of intensified nitrogen resulting from human activity. The attributed cause is dam construction induced and climate change induced reduction of river water discharge with a corresponding increase in concentration of nitrogen levels. [The answer options provided do not match increase due to water reduction resulting from climate change and water obstruction, thus your question refers to a different river-coastal water discharge system.]
Recent studies of a "coastal bay–river system in southeast China" shows "water degradation" due to a "significant decline" in the N:P ratio: there is a decrease in the N:P ratio of the river-coastal water discharge system. The attributed cause is once again anthropogenic, but this time it is specified as an unprecedented participation in China in raising livestock and in using phosphate fertilizers for crops intended for export: "proliferating husbandry of livestocks and the application of excessive phosphate fertilizers to cash crops." [Once again the answer options provided do not match decrease due to increased livestock husbandry and phosphate fertilizers, thus, again, your question refers to a different river-coastal water discharge system.]