The most appropriate option to fill in the blank is "was raining".
Explanation:
"Rained" is the simple past tense of the verb "rain". It is used to describe a completed action in the past. However, in this sentence, we are not simply stating that it rained at some point in the past. Instead, we are connecting the rain to our action of leaving the house. This suggests that the rain was happening at the same time as we left the house.
"Is raining" is the present progressive tense of the verb "rain". It is used to describe an action that is happening at the moment of speaking. Since the sentence is about an event in the past, we cannot use the present progressive tense.
"Rains" is the plural form of the noun "rain". It is not a verb and cannot be used in this sentence.
Therefore, the only grammatically correct and semantically meaningful option is "was raining". This phrase accurately conveys the idea that the rain was happening during the time we left the house.
Here's the complete sentence:
It was raining when I left the house.
This sentence effectively describes the situation of leaving the house while it was raining.