Answer:
This sentence should be a fragment.
Step-by-step explanation:
"On the desk a nail file which is used is used to open letters." I'm guessing the double use of 'is used' is a mistake.
This sentence has a subject but does not include a predicate. What will the nail file be doing? A sentence needs to have both a subject and a predicate to be complete. It is not a run-on since it does not include multiple subjects or predicates.