Question

Which of the following groups of words is a sentence fragment?
A. Stop by the garden.
B. When we stopped by the garden.
C. We stopped; she didn't.
D. Stopping by the garden, we saw that the flowers were in bloom

Answer 1

B. When we stopped by the garden.

When you stopped by the garden, what'd you do?  There's not enough information there for it to be a whole sentence.