Although it was once funded entirely by the government, the Victoria and Albert Museum was one of the first of Britain's national museums seeking support from corporations and private donors and to increase income by increasing attendance.
(A) one of the first of Britain's national museums seeking support from
(B) one of Britain's first national museums seeking support of
(C) among Britain's first national museums to seek support of
(D) among the first of Britain's national museums to seek support from
(E) among Britain's first national museums that have sought the support of

OA is D

Discussed before but I couldn't find a single post which explains why "among" is correct here. I thought when picking a part of a group, we use "of"

Eg: "The chief of Army", "He is one of my best friends"