7 * 50 = 350 ----- (1)

Since 50 is not an option, and the sum (350) consists of either 7 or 77, lets add a 77 to EQUATION 1
In order to add a 77, we need to remove 11 7s from equation 1.

so number of terms = 50 - 11 + 1 (plus 1 since we add a 77)

Answer = 40

I too had the same doubt... Great explanation Gokul...

Thanks

@chitrangada: If I am not wrong, OA according to you was E a few minutes ago.... I see that you have now changed it to D....


Can anyone please explain.......I feel D is great as per the explanation posted above...