Spot the error in the following sentence.
A. If I had known
B. this before
C. I will have helped him.
D. No error.

C

The error is in option C. The correct sentence should be:

C. I would have helped him.

Explanation for each option:

☀ A: There is no error in option A. The sentence is grammatically correct: "If I had known."

☀ B: There is no error in option B. The sentence is grammatically correct: "this before."

☀ C: The error is in option C. The correct phrase is "I would have helped him." When expressing a hypothetical situation in the past, the correct construction is "If + past perfect (had known), then + would + have + past participle (helped)."

☀ D: The statement "No error" is incorrect because the error is in option C, as explained above.