Explanation:

200 persons do 1/4 of the work in 50 days
Therefore, 1 person does 1/4 of work in 50×200 days
1 person does 3/4 of work in 3×50×200 days
n persons do 3/4 of the work in 3×50×200n
3×50×200/n = 100

n = 300
Already 200 workers are there, hence 100 more persons are to be employed.
M1×D1 /W1 = M2×D2/W2
200×50 /1/4 = (200+x)×100 /3/4
4(200×50) = 400(200+x)/3
12(10000) = 400(200+x)
300=200+x
Therefore, x = 100
Hence to complete work on time, additional employ required is 100.

Hence, option 2 is the correct answer.