Explanation:

Correct option:2

Formula Used: Total Work = Number of Men x Number of Days
Solution: Total work = 50 men x 40 days = 2000 man-days
First 10 days: => 50 men x 10 days = 500 man-days
emaining work = 2000 man-days - 500 man-days = 1500 man-days
Next 10 days: => 45 men x 10 days = 450 man-days
Remaining work = 1500 man-days - 450 man-days = 1050 man-days
Next 10 days: => 40 men x 10 days = 400 man-days
Remaining work = 1050 man-days - 400 man-days = 650 man-days
Next 10 days: => 35 men x 10 days = 350 man-days
Remaining work = 650 man-days - 350 man-days = 300 man-days
Next 10 days: => 30 men x 10 days = 300 man-days
Remaining work = 300 man-days - 300 man-days = 0 man-days
=> Total time = 10 days + 10 days + 10 days + 10 days + 10 days = 50 days
 The time to complete the work is 50 days.