Test the choices !
Pick an even number, and see what each choice does to it.
Let's start with, say, 6 .
We'll try each choice, and see which one produces an odd number:
a). 6 to the 2nd power. . . . . . 6 x 6 = 36. That's not an odd number.
b). 6 + 3 = 9 This could be it. 9 is odd. We'll save this one.
c). 3·6 = 18. That's not an odd number.
d). 6/3 = 2. That's not an odd number.
The only one that gave us an odd number is (b).