Final answer:
The selection differential is the difference between the mean of the selected individuals and the mean of the entire population, and a larger selection differential leads to stronger selection.
Step-by-step explanation:
The selection differential for a quantitative trait is the difference between the mean of the selected individuals and the mean of the entire population.
Assuming heritability is the same in all cases, a larger selection differential results in stronger selection. Therefore, the correct answer to the student's question would be 'b. The difference between the mean of the selected individuals and the mean of the entire population; results in stronger selection'.