20) C

We must look at phases of flight from descent to landing. So the order is descent, then initial approach, then final approach and then landing.

Descent takes 11% of the time out of a 1.5-hour flight.
1.5 hours = 90 minutes
11% of 90 = 9.9 minutes

Initial approach takes 12% of the time out of a 1.5-hour flight.
12% of 90 = 10.8 minutes

Final approach takes 3% of the time out of a 1.5-hour flight.
3% of 90 = 2.7 minutes

Landing takes 1% of the time out of a 1.5-hour flight.
1% of 90 = 0.9 minutes

Total time = 9.9 + 10.8 + 2.7 + 0.9 = 24.3 ≈ 24 minutes

So the answer is C.