He had a strange quirk of hitting his sister every now and then.

The correct answer is C. habit.

A fondness is a feeling of affection or liking. An aversion is a strong dislike or disinclination. A quirk is an odd or unusual habit or characteristic. A diversion is something that distracts or amuses someone, especially from something tedious or unpleasant.

In the given sentence, the man has a habit of hitting his sister. This is a negative habit that is harmful to his sister. It is not a fondness, as he does not like his sister. It is not an aversion, as he does not dislike his sister. It is not a diversion, as it does not distract him from something tedious or unpleasant.