The number $0$ is a

The number $0$ belongs to all of the nested basic classifications:
1. Whole numbers ($\mathbb{W}=\{0,1,2,3,\dots \}$)
2. Integers ($\mathbb{Z}=\{\dots ,-2,-1,0,1,2,\dots \}$)
3. Rational numbers ($\mathbb{Q}$, since it can be written as $\frac{0}{1}$)
4. Real numbers ($\mathbb{R}$)