Which of the following groups of words is a sentence fragment?
a. we stopped; she didn't.
b. when we stopped by the garden.
c. stopping by the garden, we saw that the flowers were in bloom.
d. stop by the garden.

A sentence fragment is something that is not finished, it is incomplete. Having that in mind, the correct answer is B) When we stopped by the garden, because the sentence is not finished, it is just a fragment, it needs to be supplemented with more words in order to complete it.