Fill in the blanks with an appropriate verb form:

They were playing games when it ………………………. raining.

started

The correct verb form to fill in the blank is started. The simple past tense of the verb "start" is started. The context of the sentence indicates that the rain started in the past while the people were playing games, so the simple past tense is the most appropriate tense to use.

The other options are incorrect.

• Had started is the past perfect tense of the verb "start".
• Was starting is the past progressive tense of the verb "start".
• Will start is the future tense of the verb "start".