4. However, he thought that his mother (BE) right. He needed a good night's sleep to be able to focus on the tasks tomorrow. 'OK, Mum, don't worry. I'll go to bed soon.'

Solution:

The sentence describes a past event. The verb 'BE' needs to be in the past tense, specifically 'was' because the subject is 'his mother' (singular).

Answer: 4. However, he thought that his mother was right.