The idiom 'To be thick in the head' means:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → To be stupid
The idiom to be thick in the head (or just "thick") means to be stupid, slow to understand. "Thick" suggests a head so dense that ideas cannot get in.
Example: I explained it three times, but he is too thick in the head to follow.