The correct answer is "had known."
The sentence is in the past conditional form, which means that it describes a hypothetical situation that did not actually happen. In this case, the speaker is saying that they did not clean the house because they did not know that the other person was coming.
To form the past conditional, you use the past perfect tense (had + past participle) in the if-clause and the conditional perfect tense (would have + past participle) in the main clause. So, the correct sentence is:
"I would have cleaned the house if I had known you were coming."