They both mean the same thing.
This Puertorrican right here can help you, fam.
1)Yo estudio mucho para el examen.
2)Yo miro la carretera a ambos lados antes de cruzar.
3)Yo escucho las canciones del radio.
4) Ella participa en el Día de Juegos.
5)Él necesita un lápiz.
6)Ella entrega la tarea a la maestra.
7)Nosotros trabajamos para tener buenas notas.
8)Nosotros dibujamos un cartel para la escuela.
9)Nosotros practicamos para baloncesto.
Hope this helped, fam. ;)
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
