Answer:
The correct comparatives between the dresses are:
El vestido café es más elegante del lugar.
El vestido corto es tan hermoso como el vestido largo.
El vestido largo es menos informal que el vestido corto.
Comparatives in Spanish.
The comparatives serve to mention a characteristic between two nouns, in which one of the two is better, worse or the same as the other, its structure in Spanish is usually:
Comparison of superiority.
Noun + conjugated "ser" + más + adjective + que + noun.
Comparative of equality.
Noun + conjugated "ser" + tan + adjective + como + noun.
Comparative of inferiority.
Noun + conjugated "ser" + menos + adjective + que + noun.
Therefore, to prepare comparatives adequately in Spanish, it is necessary to recognize the type of comparative to be used, select the two nouns to compare and verify their common characteristics or adjectives, following the guide above.
Explanation:
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
