Correct option is A
The underlined clause is “that virtue is its own reward,” which acts as the subject of the main verb “seen.”
Information Booster
Noun clauses stand for a person, thing, or idea and commonly answer "what?" or "who?" after the verb.
Additional Knowledge
“Adjective” and “adverb” clauses do not act as subjects or objects; the principal clause provides the main idea, which is not the case here.