Answer:
had rained
Explanation:
Past perfect tenses are used to show that an event occurred before a specific time in the past. Formation of the past perfect tense entails a combination of the past tense of the verb to have, with the past participle of the verb being talked about.
In the sentence above, had is merged with rained to form the past perfect tense. Therefore, the correct sentence is;
The ground was wet because it had rained.
