First of all, it's important to note whether the sentence is referring to plural or singular noun. It mentions "los tratamientos", therefore it is plural which eliminates A and B.
Second of all, the phrase "es dudoso que..." is commonly associated with subjunctive. Curar is the original form of the word which in subjunctive changes ar to er tenses.
Therefore your answer is C. curen because it is both subjunctive and plural
