A: That answer is illogical at best
B: It makes sense, the smaller you are, the less food you need
C: This would need extensive studying of key deer ancestry to figure out
D: This could work over thousands of years. But an overpopulation of one wouldn't push out the other. Ungulates (hooved mammals) of different species often work together.
B, sounds like your best choice.
You are correct it is the answer D.!
Water, Air (wind), Heat, Ice, Cold, Fire etc...