Gauss's law is an important principle in electromagnetism that relates the electric flux through a closed surface to the charge enclosed within that surface. It helps in understanding the behavior of electric fields and their relationship with charge distribution. The correct statement of Gauss's law is option 'D', which states that the electric flux per unit volume is equal to the volume charge density.

Explanation:
1. Gauss's law states that the total electric flux passing through a closed surface is equal to the total charge enclosed by that surface divided by the permittivity of the medium.
2. The electric flux is a measure of the electric field passing through a given area. It is defined as the dot product of the electric field and the area vector of the surface.
3. The electric field can be thought of as the force experienced by a unit positive charge placed at a point in space.
4. The charge enclosed within a closed surface can be positive, negative, or even zero. The charge enclosed is the algebraic sum of all the charges present within the closed surface.
5. The volume charge density is the charge per unit volume. It is defined as the ratio of the charge enclosed within a small volume to that volume.
6. Option 'D' states that the electric flux per unit volume is equal to the volume charge density. This means that the electric field is related to the charge distribution within a given volume.
7. This statement emphasizes the relationship between the electric field and the charge density in a given region of space.
8. Gauss's law is a fundamental principle in electromagnetism and is used extensively in the study of electric fields, electric potential, and charge distributions.

In conclusion, option 'D', which states that the electric flux per unit volume is equal to the volume charge density, correctly states Gauss's law. It highlights the relationship between the electric field and the charge distribution within a given volume.