<span>A subordinate clause has the following rules :
1) It begins with a subordinate conjunction such as { since, so that, while, when, if etc.,) or a relative pronoun such as {that, which, whom etc.,).
2) Contains its own subject and verb
3) Will not form a complete sentence in itself.
In the sentence given, only option (C) begins with a subordinate conjunction (when) and has its own subject (she) and verb (was). Also, when she was a child in itself is not a complete sentence.
Hence, the correct subordinate clause is option (C), when she was a child.</span>
