Fill in the blank with the most appropriate word. It was raining when I _________ the house this morning.

The correct answer is C. left.

The sentence is in the past simple tense, so the verb must be in the past simple form. The past simple form of leave is left.

The other options are not correct because they are not in the past simple tense.

• A. leave is in the present tense.
• B. leaving is in the present participle form.
• D. have left is in the present perfect tense.

The sentence is about a specific event that happened in the past, so the past simple tense is the most appropriate tense to use.