Which one of the following is an intensive property of a thermodynamic system?

The correct answer is Density.

In thermodynamics, the properties of a physical system are broadly classified into two categories based on how they scale with the size of the system:
1. Intensive Properties: These properties are independent of the mass, size, or volume of the system. Their value remains the same regardless of how much substance is present.
2. Extensive Properties: These properties depend directly on the size of the system or the quantity of matter contained within it.

Let us analyze why density is an intensive property while the other choices are extensive:

Density is defined as the mass of a substance per unit volume:

$$\mathrm{Density}=\frac{\mathrm{Mass}}{\mathrm{Volume}}$$
Although mass and volume are both extensive properties (since they double if you double the amount of substance), their ratio remains constant. If you take a uniform thermodynamic system and divide it into two equal halves, both the mass and the volume of each half will be halved, but the density remains exactly the same.

Consequently, mass, energy, and volume are all extensive properties because their values change when the amount of matter in the system changes. Only density remains constant, making it an intensive property.