Find out whether there is any grammatical error in below sentence. Had I / known it earlier / I would contact you.

The correct answer is D. No error.

The sentence “Had I known it earlier, I would contact you” is grammatically correct. It is in the past perfect subjunctive mood, which is

used to express a condition that was not true in the past. The past perfect subjunctive is formed with the past participle of the verb (known) and the auxiliary verb had.

The sentence can be paraphrased as “If I had known it earlier, I would have contacted you.”