Final answer:
The correct conclusion from the Grant's study is that both the strength and direction of natural selection on beak sizes of finches vary over time, influenced by changes in the environment.
Step-by-step explanation:
From the Grant's long-term study of the medium ground finch on Daphne Major, we can conclude that the strength of selection varies over time and the direction of selection varies over time. This is due to the fluctuation of food resources, such as changes in seed availability, which affect the survival and reproduction of finches with different beak sizes. However, it cannot be concluded that beak size should continue to get larger over time, as beak size changes in response to the available food sources and the prevailing environmental conditions. Therefore, the correct answer is D) a and b are correct.