The error in the sentence "One should respect his elders" is D: his. This is a common mistake related to pronoun agreement. When the indefinite pronoun "one" is used as the subject of a sentence, the possessive pronoun referring back to "one" should also be "one's," not "his," "her," or "their." The pronoun "one" is used to refer to people in general, and using "one's" maintains this general, impersonal tone throughout the sentence.
Using "his" (or "her") makes the sentence sound as if it's referring to a specific male (or female) person, which contradicts the general nature of "one." While some older grammar conventions might have accepted "his" as a generic pronoun, modern English grammar emphasizes gender-neutrality and consistency. Therefore, to maintain proper pronoun agreement and a general reference, the sentence should be "One should respect one's elders." This ensures that the pronoun correctly reflects the antecedent and maintains the intended meaning of the statement.