If the grasshopper population had a major increase, the most likely response would be that the frog population would decrease. This is because an increase in grasshoppers would lead to more food availability for frogs, potentially causing them to initially thrive. However, long-term effects might include overpopulation of frogs leading to competition for resources or increased predation, which could eventually lead to a decrease in the frog population once other factors come into play.
Thus, the most immediate direct impact seems to be that the frogs, which eat grasshoppers, might benefit initially, but this doesn't guarantee that their population will remain stable or increase indefinitely.
So the best option among the responses provided would be:
The frog population would decrease.
However, keep in mind that ecosystems are complex, and other factors would also play a role over time.