Stay with A.
The metabolism of X is compared to a mammal's (metabilism) and reptile's (metabolism)
C & D out 'coz "metabolisms" .....
B out 'coz metabolism .... were is incorrect
E. a metabolism more like that of a mammal's (metabolism) is wrong. It should either be metabolism more like that of a mammal.
"To dream anything that you want to dream, that is the beauty of the human mind. To do anything that you want to do, that is the strength of the human will. To trust yourself, to test your limits, that is the courage to succeed."
- Bernard Edmonds