Answer:
Trout population
Step-by-step explanation:
In any scientific study you will have at least one independent and one dependent variable. Independent variables do not change in function of other variables in the study, while dependent variables change when the independent variable changes. In the proposed study, if the scientist wants to find out the impact of trout population on the yellow-legged frog population, what they really want to find out is how much the frog population changes when the trout population changes. This means that the trout population doesn't depend of any other variables in the study, so it's independent, while the frog population depends on the trout population, so it's dependent.