Distinguishing one thing out of many

Jamieson's proposal was rejected for several reasons, the chief among which was cost.
(a) same
(b) among which the chief was its cost
(c) the main one was cost
(d) the chief reason of which was its cost
(e) the chief of which was cost

When you want to point out to one particular thing out of a group of things, use "of" to distinguish.

Ans E