Explanation:

Persons worked (M1) = 104
Number of hours each person worked per day (H1) = 8 
Number of days they worked (D1) = 30
Work completed (W1)= 2/5 
Remaining days (D2)= 56 - 30 = 26
Remaining Work to be completed (W2)= 1 - 2/5 = 3/5
Let the total number of persons who do the remaining work (M2) = x
Number of hours each person needs to be work per day (H2) = 9 

M1D1H1W1 = M2D2H2W2 ⇒ 104×30×8 (25) = x×26×9 (35) 

⇒104×30×82 = x×26×93 

⇒52×30×8 = x×26×3

⇒2×30×8 = 3x

⇒x = 2×10×8 =160

Number of additional persons required = 160 - 104 = 56

Hence, option 2 is the correct answer.