Retirement is obviously a very complex ______ period; and the earlier you start planning for it, the better.

【正确答案】 A

【答案解析】 选项中的词以trans-开头。transition“(从一种状态到另一种状态等的)转变，过渡”，符合句中retirement的状态。故答案为A。transaction“交易”；transmission“传播”；transformation“改变，变化”。