Correct option:
had known, have visited ✅
-> Third conditional (imagining the past): If I had known, I would have visited.
Conditionals Recap:
Zero Conditional
Use: Facts, general truths
Structure: If + present simple, present simple
Example: If you heat ice, it melts.
First Conditional
Use: Real future possibility
Structure: If + present simple, will + base verb
Example: If it rains, I will stay home.
Second Conditional
Use: Unreal or imaginary present/future
Structure: If + past simple, would + base verb
Example: If I had wings, I would fly.
Third Conditional
Use: Unreal past (something that didn’t happen)
Structure: If + had + past participle, would have + past participle
Example: If I had studied, I would have passed.