Which one of the following is not a heterogeneous mixture?

A mixture is homogeneous when its composition and properties are uniform throughout the sample, forming a single phase (for example, a true solution or a mixture of gases). A mixture is heterogeneous when it contains two or more visibly or microscopically distinguishable phases -- such as a colloid, an emulsion, or a suspension -- even if it looks uniform to the naked eye.

Among the given substances, three are colloidal systems with two distinguishable phases, and only one is a genuinely single-phase gaseous mixture.

• Smoke: fine solid (ash/carbon) particles dispersed in air form a solid-in-gas colloid (an aerosol) -- two distinguishable phases, so it is heterogeneous.

• Milk: tiny fat globules dispersed in a watery medium form a liquid-in-liquid colloid (an emulsion) -- two distinguishable phases, so it is heterogeneous.

• Jelly: a solid network phase traps liquid within it, forming a gel -- a solid-liquid colloidal system with two distinguishable phases, so it is heterogeneous.

• Air: nitrogen, oxygen, and other gases are mixed uniformly at the molecular level with no separate phase boundary -- a single-phase gaseous mixture, so it is homogeneous.

Since air is the only single-phase, uniformly-mixed substance among the four, it is the one that is not a heterogeneous mixture.