Answer: Choice A
Step-by-step explanation:
A complete sentence must have a subject and a predicate (verb), and a complete thought (It has to make sense).
In Choice A, there is a subject (we) and a predicate (stopped) , but the sentence does not have a complete thought. (This is a dependent/subordinate clause)
Choice B has a predicate (stop), and there is an implied “you” in the beginning of the sentence. “(You) stop by the garden.”
Choice C is 2 complete sentences separated by a semicolon.
Choice D has a subject (we), a predicate (saw), and a phrase and a clause.