In any experimentation, we are concerned with independent and dependent variables. The independent variable is the parameter that we change and dependent variable is the one that gets affected by changes in independent variable.
In the present case, we are interested in determining the amount of time it will take for salt to get dissolved. The missing information is whether we are interested in varying amounts of salt or varying temperature conditions. That is, the objective is the time taken for different quantities of salt to get dissolved or time taken for salt to get dissolved at different temperatures. With this information and definitions of independent and dependent variables, we can state that either of the mass of salt or temperature of mixing is the independent variable, while the time required is the dependent variable (since it depends on the mass of salt or the temperature).
Hope this helps.