Zero is an even number.
This question might seem tricky at first glance. But there is a detailed and clear explanation, which proves that zero is, in fact, an even number:
Zero is an integer (in other words, a whole number, like 1, 3, 54, -40). An even number is an integer multiple of 2 (so 6 is even because it is 3 x 2; 2 is also an even number because it is 1 x 2). So zero is also an even number because it fits the definition of "even": it is an integer multiple of 2, specifically 0 × 2.
Equivalently an even number is an integer which, when divided by 2, leaves no remainder: it is clear that this is true in the case of 0.