24) C

The process of exhalation or expiration is as follows:

  1. volume of thorax decreases (rule out options B and D)
  2.  pressure in thorax increases since volume decreases
  3.  diaphragm relaxes and becomes more convex (rule out option E)
  4.  ribs come down and inwards (rule out option A)

If you face any confusion, try exhaling yourself and feel the movement of your ribs. Also, you will also notice that when you are about to exhale, the pressure inside your lungs increases. Since pressure is inversely proportional to volume, volume would decrease.