a) may; b) must; c) should.
1. Let's wait for her. She (возможность) be on her way.
2. You (следовало) have started the treatment earlier.
3. You (нельзя) not smoke.
4. The doctor (должно быть) be making a round of the district.
5. Your sister often complains of a headache. She (следует) consult a neurologist.
6. This patient (возможно) have already undergone the necessary examination.
7. One (нужно) stay in bed and follow the prescribed treatment not to get complications.
8. (Можно) I listen to your heart?
9. You (должен) take the medicine regularly to be well again.
10. You (должен) begin to take care of your health from childhood.