Correct Answer: supine

Solution : The fourth option is correct.

Supine means lying on the back with the face upward. In a medical context, it often refers to a position where the back is resting against a surface, which is likely the case on a treatment couch.

The meanings of the other options are as follows:

• Arsine is a toxic gas containing arsenic.
• Alpine is related to high mountains, especially the Alps.
• Repine means to express discontent.

Therefore, the complete sentence is, "During treatment, patients lay supine on the treatment couch for around 3 hrs".