Fill in the blank with a suitable modal. Had I seen you, I _________…

The correct answer is Option (3) → D only

Third conditional with inversion:

• "Had I seen you" is the same as "If I had seen you": the if-clause of the third conditional with "if" dropped and "had" put before the subject.
• The main clause of the third conditional takes would / could / might + have + past participle.
• This sentence: "Had I seen you, I would have come."

Complete sentence: Had I seen you, I would have come.

A. will — Incorrect.

• "Will have come" is the future perfect.
• It cannot express the result of an imagined past.

B. shall — Incorrect.

• "Shall" too refers to the future and does not fit an unreal past.

C. can — Incorrect.

• "Can" is a present form.
• The past form could would be needed (I could have come).

D. would — Correct.

• "would have come" gives the result that did not happen.

Tip: In this question the options do not follow the order of the statements (Option 1 = A only, Option 2 = B only, Option 3 = D only, Option 4 = C only). NTA often jumbles the options in this way, so after finding the correct statement, read the options carefully and mark the one with the right letter.