Final answer:
The well-written sentence is B. 'The set of concepts was explained.'
Step-by-step explanation:
Out of the given options, the sentence that is well-written is B. 'The set of concepts was explained.' In this sentence, the subject 'set of concepts' is singular, and the verb 'was explained' agrees with it. A well-written sentence should have subject-verb agreement, where a singular subject is paired with a singular verb, and a plural subject is paired with a plural verb.