Since you have brought that up.
This is a sentence.

The correct answer is 'False'.

A sentence fragment is a sentence that is missing either its subject or its main verb. Fragments are incomplete sentences. Usually, fragments are pieces of sentence that have become disconnected from the main clause. 'Since you have brought that up', it is sentence fragment. It needs an independent clause to give full meaning where it is a dependent clause. So, the above statement is 'false'.