Explanation:

Efficiency — A : C = 120 : 100 = 6 : 5
So days ratio = 5 : 6
Similarly, A : B = 60 : 100 = 3 : 5, so days 5 : 3
B/A = 3/5, A/C = 5/6
So ratio of days for B : A : C is 3 : 5 : 6 …………(1)
Now — B and C can complete the whole project 4 in 6 days working together,

and ratio of no. of days in which they can complete work alone is B and C from (1) is 3 : 6 = 1 : 2
So
1/x + 1/2x = 1/6
Solve, x = 9
So B can complete work in 8 days, C in 18 days and then A in 15 days
Now, let D worked for ‘y’ days on project 4. So
1/15 *5 + 1/9 * 3 + 1/18 * 2 + 1/18 * y = 1 – 1/6
Solve, y = 1 day

Hence, option 4 is the correct answer.