The term 'smog' is a combination of two English words: "smoke" and "fog." Let's break down the elements of this term to better understand its meaning:

• Smoke: This refers to the visible suspension of carbon or other particles in air, typically as a result of burning.
• Fog: This is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface, which obscures or restricts visibility.

When these two elements combine, especially during the industrial era when a lot of smoke from factories was mixed with regular fog, a new phenomenon called "smog" was observed. Smog is characterized by a thick, often brownish haze in the atmosphere, commonly found in urban environments, where industrial and vehicular emissions are significant.

Now, let's analyze why the other options are not correct:

• Fog and clouds: These are both forms of condensed water vapor, but clouds typically form higher in the atmosphere, and the combination does not specifically refer to smog.
• Smoke and haze: While smoke can contribute to haze, "haze" generally refers to the suspension of extremely fine particles or moisture droplets in the air, which is different from fog's moisture concentration needed for smog.
• Smog and mist: This is incorrect because "smog" cannot be part of its own definition. Mist is similar to fog but generally has a shallower appearance, and this combination does not define a new phenomenon.

The correct answer is, therefore, the combination of "Smoke and fog."