Complete the correct option 4 When their mother called, the children PLAYED | WERE PLAYING games.

The correct answer is WERE PLAYING.
Explanation: The sentence describes an action (playing games) that was in progress when another action (mother called) occurred. 'Were playing' is the past continuous tense and is used to show that the children were in the middle of playing when the mother called. The actions of playing and mom call are in progress simultaneously. so, we use the past continuous.