The idiom 'To be thick in the head' means:

To be stupid

The correct answer is Option (3) - To be stupid

Explanation: This idiom refers to someone who is slow to understand things or lacks intelligence. It suggests that the person is mentally "dense" or not quick-witted, often in a way that leads them to make poor decisions or overlook obvious facts. The phrase uses the metaphor of "thickness" in the head, implying that the person's mind is not sharp or clear. It is not typically used to describe someone who is irrational or angry, but rather someone lacking in mental acuity.