The product of a

Let the non-zero whole number = a

The successor (number after) = a + 1

Case 1:

If a is even,

Successor = a + 1 is odd

The product of an even and odd number is always even.

(For example: 2 × 3 = 6)

Hence, the answer is an even number.

Case 2:

If a is odd,

Successor = a+1 is even

The product of a even and odd number is always even.

(For example: 5 × 6 = 30)

Hence, the answer is an even number.