Has got to be A as it is all in past tense...
IMO A

limestone wrote:It was primarily when I visited the seashore that I felt nostalgic for my childhood home, growing up on a small island.

A.
B. as I had grown up on a small island
C. as on a small island I grow up
D. which is on a small island
E. on a small island growing up

[spoiler]OA: B[/spoiler]

IMO B

If I am looking for main subject and verb by eliminating adverbial clause, what I get - "It was".

A. The participle is modifying the subject "it" - Wrong
B. Correct. Also the tense is correct.
C. Tense error. Wrong
D. Adjective clause is modifying home. Wrong
E. This prepositional phrase has not proper referent. If we assumes that it were functioning as adjective, then it would modify "home" - the nearest noun. However, if it were functioning as adverb, then it would modify "felt" or "visited". In any case, both of them are wrong.

Could someone please explain the options A and B in detail ?