Find the missing number?

Answer: Option D

Working in rows, add the left and right hand values together and subtract 3 to give the number in the center of each row. So,

Row wise : 1st column number + 3rd column number - 3 = 2nd column number
1st row : 7 + 8 - 3 = 12
2nd row : 1 + 6 -3 = 4
Similarly last row : 12 + X - 3 = 13.
X = 4.