Question

Which choice explains one way to correct a relative clause​ fragment?

A.
add a verb to the fragment
B.
reword the fragment to make it an independent clause
C.
change the fragment into a dependent clause
D.
use a conjunction to attach it to the main clause

Answer 1

the choice that explains one way to correct a relative clause fragment could be option D (use a conjunction to attach it to the main clause.
There are plenty of conjunction that can be used but it is important to pay attention to the content of the clause in order to choose one of them.

Conjunctions (some examples)
After- how- if - that/which- since - where- because, etc

Answer 2

I would say one answer could be to use a conjunction to attach a relative clause to the main sentence. An example could be that, say, "He was so heavy he could not walk and needed a wheelchair" which could be changed to "that he could not walk and needed a wheelchair".