Select the most appropriate option to substitute the highlighted segment in the given sentence. In case no substitution is needed, select 'No substitution required'.
I would wait for you at the Metro station had I known that you would surely come.

Correct option is A

The correct substitution of the highlighted words is (a) would have waited.
Explanation:
The sentence is a third conditional structure which is used to express an unreal past condition and its probable result in the past. The correct structure is:
If + past perfect, would have + past participle
In this case, “had I known” = past perfect form, so the result clause should be “would have waited.”
Hence, “I would have waited for you at the Metro station had I known…” is the grammatically correct sentence.
Grammatical rule used:
Third conditional sentences express something that did not happen in the past and its imagined result.
Structure:
· If + Past Perfect, would have + V3
· Example: If she had studied harder, she would have passed the exam.
· Information booster:
· Using “would + base verb” with a past unreal condition (like “had I known”) is incorrect. Use “would have + V3” instead to match the unreal past.