Question

Though no longer a series of colonies, the United States retains close ties to Britain as

Answer 1

Though no longer a series of colonies, the United States retains close ties to Britain as part of a “special relationship
So the answer is d.

Answer 2

Though no longer a series of colonies, the United States retains close ties to Britain as "d. part of a “special relationship”" since trade between these two nations benefits both immensely.
Answer from "HistoryGuy" on another user's same question.  Not from me.