"in year three, the farmer discontinued the use of the insecticide. which statement would best predict the population in generation 4?
(1)the nonresistant aphid would become extinct.
(2)the nonresistant aphid population would likely increase.
(3)the resistant aphid would mutate to a nonresistant aphid.
(4)the plants would be free of insect populations."

After looking at the graph represented above, I think I habe recognized the correct answer,  so the statement  which would best predict the population in generation 4 is being shown in the first option : The nonresistant aphid would become extinct. As you can see from the tree pictures, nonresistant ones gradually become lower and lower and in the last one it has the same weak position as resistant in the first picture, then I would say that nonresistant will expire soon.

Hope that helps!

Mg is used to measure mass

No you are not wrong