Many people died in London in 1952 due to air pollution producing:

The devastating air pollution event in London in December 1952, which led to thousands of deaths and severe health issues, is historically known as the Great Smog. This phenomenon was a thick, toxic mixture of smoke and fog, primarily caused by extensive coal burning for heating and industrial emissions, trapped by a temperature inversion.

Therefore, Smog is the correct answer. Option A, 'Mist,' refers to fine water droplets in the air, which are generally harmless. Option C, 'Fog,' is a cloud at ground level, primarily water vapor, and while it was a component, the toxicity came from the smoke. Option D, 'Sleet,' is a form of precipitation consisting of ice pellets, unrelated to air pollution in this context.