You answered your own question. The number zero is an even number. There are no arguments or different opinions about this.
People's confusion may come from different questions regarding zero. For example, is zero positive or negative? It is neither.
Another example: if we are not talking about the number zero, but the zero function (which is, a function $f(x)$ that maps to $0$ for every input value $x$), and we ask whether the zero function is an even function or an odd function (if you need to know the definition of even function and odd function, search Wikipedia). The answer to this question is: it is both. But this is a completely different question from the question about the number zero.