The bottleneck event that decimated the bison was caused by . a. overhunting
b. climate change
c. disease
d. habitat destruction

D

Explanation:

Although the bison were killed off largely due to a mix of A and D, I think a bottleneck event is usually a environmental issue, so I'd go with D.

goog may have it for you

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.