Read the given sentences and fill in the blanks with the appropriate type of verbs mentioned in brackets.

It ____ raining heavily.

The correct answer is: Was

Past continuous tense can be formed in the following way: subject + was/were +v-ing. Whereas 'was' is the singular past tense of to be, were is used for both the third person plural past tense (they and we) and the second person past tense (you).
'It was raining heavily', here 'it' is third person singular that's why used 'was'.