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It doesn't imply he did it only once. He could заходить to his sister many times in the past against father's will, but the focus of the sentence is on that particualr visit.

The question is ok, the answers are ambiguous. A) and C) are possible. I still believe answer A) is the best (keeping in mind the way tests are usually composed).

They are not all perfectly correct. They may be correct with some additional condition or in some context. For example "заходил" is normal when it's obvious from context that it was a repetetive...

A) зашел