Identify the part of the sentence which has an error in it.
He told (A)/ me yesterday (B)/ that he (C)/ will help me. (D)

Correct option is A

Option (D) contains an error.
Detailed explanation of error: The reporting verb told is in the past, so the verb in the reported clause should undergo backshift: will → would. Correct: “He told me yesterday that he would help me.”
Grammatical rule used: Sequence of Tenses in indirect/reported speech—when the reporting verb is in the past, future will generally changes to would, present to past, etc., unless the fact is timeless.
Example: Direct: He said, “I will help you.” → Indirect: He said that he would help me.
Booster: No backshift occurs for universal truths (“He said the sun rises in the east”) or if the reporting verb is present.