Given the sentence "Our aunt Helen participated in a medical study that resulted in the cure.", the correct answer to which word does the boldface clause modify is D. Study.
The dependent clause "that resulted in the cure" starting with the subordinate conjunction "that" is a restrictive relative clause. This kind of clause does not function as an noun phrase but it has an adjetival use. This means that it contains a reference to a nount phrase. This eliminates both options B and C since they are a verb and an adjective.
Even if Aunt Helen was cured, she is not the word modified by the clause. The boldaface clause modifies the word study because if completes the information by stating that it was THAT particular study what cured resulted in a cure. The clause functions as an adjective that modifies the word study.