On her way home,Mary saw a thief----（steal\ stealing) in a shop.She called 110 at once.

On her way home,Mary saw a thief----（steal\ stealing) in a shop.She called 110 at once.
On her way home,Mary saw a thief----（steal\ stealing) in a shop.She called 110 at once.

On her way home,Mary saw a thief----（steal\ stealing) in a shop.She called 110 at once.
see sb do sth是指看到某人做某事的全部过程,指从开始到结束一直看到
see sb doing sth是指看到某人正在做某事,没有看到全过程.
这句话应该是选stealing