This sentence requires subject-verb agreement within a relative clause.
The subject of the verb “believe” is “those,” which is plural. Therefore, the verb must also be plural — “believe” — not “believes.”
Option (A) is correct: “those who believe everything they hear” keeps the verb in plural form to match “those.”
Option (B) uses “believes,” which mismatches with the plural “those.”
Option (C) also uses “believes” incorrectly and changes the pronoun to “he,” which unnecessarily limits the general meaning.
Option (D) uses “believes” and shifts the meaning by adding “in,” changing the sense from accepting information as true to having faith in something, which is slightly different.
Thus, (A) is grammatically correct and semantically accurate.