What number is considered to be a whole number?

Whole numbers are defined as the set of non-negative integers that do not include any fractions or decimals. They start from zero and continue indefinitely in the positive direction: 0, 1, 2, 3, and so on.

In this context, the number zero is indeed classified as a whole number. It's the smallest whole number and serves as the foundation of the whole number system. The other numbers provided in the options either fall into negative integers or represent fractions or decimal values, which disqualifies them from being categorized as whole numbers. Thus, zero is the only number in the given choices that fits the definition of a whole number, confirming that the correct answer is zero.