The expression that best completes the sentence “If he had left home earlier, he … late” is would not have been.

This phrase indicates that if the condition (he had left home earlier) were met, then the result (he would not have been late) would have occurred.

Here’s why the other options are not correct:

• would not be: This phrase changes the tense from past to present, which doesn’t fit well in this context because the sentence is referring to a hypothetical past event.
• would have not be: This phrase is grammatically incorrect. The correct phrase would be “would not have been.”
• would not been: This phrase is grammatically incorrect. The correct phrase would be “would not have been.”
• would have been not: This phrase is grammatically incorrect. The correct phrase would be “would not have been.”