For a number to be divisible by 9, the sum of the digits of the number msut give a multiple of 9
3 + 7 + 1 = 11
The missing number when added to 11 must give a multiple of 9. Considering all the options
3 + 11 = 14. This is not a multiple of 9 hence 3 is not the number
7 + 11 = 18. This is a multiple of 9 hence 7 is the right option
