Answer:
The answer is: C) that Sara wrote.
A clause includes a subject, verb and sometimes a complement. As a result, an adjective clause must contain a relative pronoun or adverb, and functions as an adjective, thus modifying a noun and providing further description.
The rest of the options are incorrect because "The report" is a noun; and the expressions "on the plight" and "of poor children in Haiti" are prepositional phrases which lack a subject and a verb.