15. As the smallest child of his family, Alex is always longing for the time __________ he should be able to be independent.
A
which
B
where
C
whom
D
when
这里使用定语从句，先行词是the time，定语从句中缺少时间状语，用关系副词when，。故选项D符合答案。
根据句意：作为家里最小的孩子，Alex总是渴望他能够有独立的时间。。
对定语从句不熟悉。