Answer 1

Here operation in printf() executed as:
prod(x+2,y-1)
prod(3+2*4-1)
prod(3+8-1)
prod(3+7)
prod(10)

In this way output is 10

Answer 2

prod(x + 2, y - 1) = prod(3 + 2, 4 - 1) = 3 + 2*4 - 1 = 3 + 8 - 1 = 10

Answer 3

Note that the behavior of the prod(i, j) may not be as expected, to fix this, maybe you can state:
#define prod(i, j) (i)*(j)
This example shows that the define should be used very carefully.

Answer 4

It happens because multiplication has higher precedence than sum. It is often tricky using preprocessor functions