He told the members of his staff that each one should carry out his task oneself.

Option D, “oneself”, is incorrect in this context. The pronoun “oneself” does not agree with the previous pronouns “he” and “his”.

In English, pronoun agreement is a grammatical rule that states that a pronoun must agree in number, gender, and person with its antecedent. In this case, the antecedent for “oneself” is “each one”, which is referred to with “his” earlier in the sentence. Therefore, the correct pronoun should be “himself” to match with “he” and “his”. So, the corrected sentence would be:

“He told the members of his staff that each one should carry out his task himself.”