Explanation:

Correct option 1: 5

We are given:

• A man can complete the work in 3 days → So, 1 day's work = 1/3

• A woman can complete the work in 4 days → So, 1 day's work = 1/4

• A boy can complete the work in 12 days → So, 1 day's work = 1/12

We are to find how many boys (say x) are needed to assist 1 man and 1 woman so that they together complete 1 full work in 1 day.

Total work done in one day=

Man's work + Woman's Work + Boy's Work 

1/3+1/4+x.1/12=1

Solving of  =n we get x =5