Which of the Statements Below Is the Most Reasonable Explanation

Which of the statements below is the most reasonable explanation for why many mammals, such as deer, give birth in the spring?

A) This is a result of all mammals possessing lunar clocks.
B) This is the result of all mammals possessing annual clocks.
C) This is unique to a subset of mammals that possess circadian clocks (most animals don't actually have these clocks) .
D) This is the result of certain mammals having seasonal clocks dependent on photoperiods.
E) This is the result of mammals responding to changes in Earth's magnetic field.