Roberto pretended to know a lot about the opera, but he was really just a ___________ .

To solve this question, the user needs to know the meanings of the given options and the context of the sentence. The sentence implies that Roberto pretended to know a lot about the opera, but in reality, he did not know much about it.

Explanation of each option:

A. dilettante: This option means a person who loves or appreciates the arts, especially someone who is not a professional. This option fits the sentence and describes Roberto's behavior.

B. supernumerary: This option means a person who performs in a theatrical production without a speaking role. This option does not fit the sentence, and it has no relation to Roberto's behavior.

C. chimera: This option means a thing that is hoped for but is illusory or impossible to achieve. This option does not fit the sentence, and it has no relation to Roberto's behavior.

D. catalyst: This option means a person or thing that causes a change or event to happen. This option does not fit the sentence, and it has no relation to Roberto's behavior.

The Answer is: A. dilettante