Find the missing number?

Answer: Option A

Working from top to bottom,
The sum of the numbers in each row is put in the left hand box on the row below, and the difference between the two numbers in each row is put in the right hand box on the row below. I.e.

Row one First Element + Row one second element = Row two first Element.
Difference of first row elements = Row two second Element.
And So on.

Thus,
Row fifth first element + row fifth second element = Row sixth first element
32 + 12 = 44
And,
32 - 12 = 20 ( Row sixth second element).