In Bilaspur village, 12 men and 18 boys completed construction of a primary health center in 60 days, by working for 7.5 hours a day. Subsequently the residents of the neighbouring Harigarh village also decided to construct a primary health center in their locality, which would be twice the size of the facility build in Bilaspur. If a man is able to perform the work equal to the same done by 2 boys, then how many boys will be required to help 21 men to complete the work in Harigarh in 50 days, working 9 hours a day?

It is given that a man is able to perform the work equal to the same done by 2 boys. Hence, we can assume that In Bilaspur village 21=(12+18/2) men completed construction of a primary health center in 60 days, by working for 7.5 hours a day. 

Hence, total men hours required to completed the primary health center in Bilaspur = 7.5*21*60 = 9450. 

Let us assume that 'n' be the number of boys required to help 21 men to complete the work in Harigarh in 50 days by working 9 hours a day. 

Therefore, we can say that

$$\Rightarrow$$ 2*9450 = 50*9*(21+t/2) 

$$\Rightarrow$$ 42 = 21+t/2

$$\Rightarrow$$ t = 42 

Hence, option D is the correct answer.