Explanation:

The ratio of the efficiencies of A, B and C is 3 : 5 : 1
Let the work done by A, B, C is 3 units, 5 units, 1 unit in 1 day respectively.
Work done by A,B,C together in 1 day = 9 units
Working together, A, B, C can complete a piece of work in 5 days.
Total work = 9 x 5 = 45 units
A and B work together for 3 days.
Work done by A and B together in 3 days = (3+5) x 3 = 24 units
Remaining work = 45 - 24 = 21 units
Remaining work will be completed by C who can do 1 unit per day.
Number of days required for C to complete remaining work = 21 days
Hence, the correct answer is Option D