Answer : Option D

Explanation :

3M + 4C earns 756 in 7 days
Or, (3M +4C)'s one day earning = 756/7 = 108........ (i)

Similarly,

(11M + 13C)'s one day earning = 3008/8 = 376......... (ii)

From equation 1 and 2

3M + 4C = 108
11M + 13C = 376
Multiply equation 1 by 11 and equation 2 by 3 to make the M equal.

11* [3M+4C] = 108 * 11, or, 33M + 44C = 1188....... (iii)

Now subtract equation iv by iii.

i.e., 5C = 60
Or, 1C = 12

That means 1children earns 12 rupees per day
Now, put the children's one day earning, in equation i or ii

3M+ 4C=108
3M + 4 * 12 = 108 or, 3M = 108-48
Or, M = 60/3 = 20

That means 1M earns 20 rupees per day

ATQ, 7M + 9C earns 2480 in D days........ (v)

Now, put the earned value by M and C per day in equation 5.

7*20 + 9*12 = 248
But they earn 2480, i.e., multiply the one day earning by 10, and we get 2480
So, they require 10 days to earn 2480.