Correct option is D
Given: A and B can complete a work alone in 20 days and 60 days respectively. B completed the remaining work in 12 days.
Formula Used: Work done = (Rate of work) × (Time)
Solution:

Work done by B in last 12 days = 12 × 1 = 12 work.
Remaining work done by A + B = 60 – 12 = 48 work
A left the work after = 48 / 4 = 12 days.