People are told not to drink the water in Mexico because they may get "Montezuma's Revenge." This is confused with amoebic dysentery and they are not the same thing. People who get sick in Mexico usually get sick from bacteria or viruses that are present in the water due to improper water and sanitation systems.
Amoebic dysentery is caused by amoebae which form cysts and then spread. They are found in the stools of contaminated individuals and are spread not only through water but food as well. Basically, a contaminated individual goes to the bathroom and does not wash their hands or they wash them improperly. They then touch food or water that another individual eats and that is how it is spread.
If these are your two choices for answers, the correct choice is A -- amoebic dysentery.
It is a pretty common stereotype that you must not drink the water in foreign countries so that you do not get diarrhea. Dysentery is a type of sickness in which you get diarrhea, so that is one reason this is the correct answer.
The other way you know which of these is correct is that malaria cannot possibly be correct. The only way to get malaria is to be "bitten" by a mosquito that carries the disease. You cannot get malaria by ingesting food or drink.
I find the question and the choices to be oversimplified and cliched. Firstly, what exactly do "foreigners" mean? From the perspective of the person who is asking the question, would someone from Nicaragua be considered as much as a foreigner as someone from the United States while visiting Mexico? Furthermore, who is the one formulating the question and for what purposes?
Many people who travel from one region to another, even within the same country, develop new diseases or a strong ailment from viruses or bacteria strains to which an individual has not yet developed full immunity. For instance, people who move from Toronto to Vancouver, and vice-versa, tend to get unusually strong colds the first year of having settled in the new region. With regard to "the water" in Mexico, all cities in Mexico have potable water that is up to international standards. Are foreigners who are getting sick mostly those who go to questionable resorts designed for international tourists, or are they purchasing food and beverages from street vendors or restaurants that cut corners in terms of hygiene?
This leading question also lacks grammatical logic. People need to drink water on a daily basis. Not drinking water while visiting a foreign country (for several days at least) would be far more harmful (and deadly) than drinking it.