If I had known, I … not have come.

Step 1: Analyze the question The sentence is a conditional sentence: "If I had known, I ......... not have come." We need the modal verb that correctly completes it.

Step 2: The concept: Third Conditional (Type 3) This type talks about an unreal or imaginary situation in the past, something that did not happen.

Structure: If + past perfect (had + past participle), subject + would/could/might + have + past participle

• If I had known (past perfect), I would have come.

Step 3: Test the options

• A. should: "I should not have come" is grammatically possible, but it means "it was wrong of me to come," which is advice or regret. It does not express the result of an unreal condition as naturally as "would."
• B. ought: Needs "to" ("ought not to have come"), so it doesn't fit grammatically.
• C. would: "I would not have come" is the standard result clause in the third conditional. It means: because I didn't know, I came, but if I had known, I wouldn't have. ✔
• D. may: "may not have come" expresses possibility or uncertainty, not a firm result of the condition.

Step 4: Answer The correct option is C. would.

Tip: In a Type 3 conditional, the "if" clause uses had + past participle, and the main clause uses would have + past participle. Remember: "had" in the if-clause, "would have" in the result.