单选题

A He saw them during his visits to Britain.

B They were increasing in numbers in London.

C He studied them at school.

D He once saw them eating.

解析：
对话中Jim Clark提到当他还是个孩子的时候他就对狐狸感兴趣，那时他们刚开始从乡村移向伦敦，进行城市殖民化，他看到一种野生动物在城市里建立自己的人口深深地吸引着他(seeing a wild animal build up its population in the city really fascinated me)，由此可见，Jim Clark对狐狸感兴趣的原因是它们在伦敦增加数量，故答案选A。