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

Sentence fragment is defined as the set of words that look like sentences but actually they are not.For a sentence there should be a absolute clause.
therefore,
first and second option have absolute clause while third one is sentence fragment.
hence the right option is C) When we stopped by the garden.

Ibelieve the answer is d