Which one of the following is not a heterogeneous mixture?

Let's analyze the properties of the mixtures listed:
Heterogeneous Mixtures: These mixtures have non-uniform compositions, and their individual components can be distinguished at a microscopic or macroscopic level. Milk, Smoke, and Jelly are colloids, which are classified as heterogeneous mixtures.

Homogeneous Mixtures: These mixtures have a uniform composition and appearance throughout their volume. Air (when clean and free of dust or pollutants) is a stable, uniform solution of gases (mostly Nitrogen and Oxygen), making it a homogeneous mixture.