Answer:
Sentence fragment
Step-by-step explanation:
She remembered to buckle her seatbelt. Which is always a good idea.
Sentence fragment is when a sentence doesn't specifically state a full thought, which in this case the sentence doesn't state a full thought.
What happened after she remembered to buckle her seatbelt? The sentence ended and cut straight to it being a good idea to put on her seatbelt. Why is it a good idea to put on her seatbelt?
The sentence does not state a full thought, which means your answer has to be the first option or "sentence fragment."
Hope this helps.