D. 37 1/2 days
Feedback A alone can finish the work in 20 × 5/4 days = 25 days
A’s 1 day’s work = 1/25
A and B together can finish the whole work in 5 × 3 days = 15 days
Their 1 day’s work = 1/15
Therefore, B’s 1 day’s work = 1/15 – 1/25 = 1/37 ½
Therefore, B alone can finish the whole work in 37 ½ days