The correct answer is: D. Water drops in a dispersion medium of air.
A colloid is a mixture of two or more substances where one substance of microscopically dispersed insoluble particles is suspended throughout another substance. The particles of the dispersed phase have a diameter between 1 and 1000 nanometers. The dispersed phase is called the colloidal phase, and the medium in which it is dispersed is called the dispersion medium.
Clouds are formed when water vapor in the air condenses around tiny particles of dust or salt. These particles act as nuclei for the water droplets to form on. The water droplets are so small that they remain suspended in the air, and they reflect and scatter sunlight, which is why we see clouds.
Option A is incorrect because air is the dispersion medium in a cloud, not the dispersed phase.
Option B is incorrect because fog is a type of cloud that forms when water vapor condenses in the air near the ground. The water droplets in fog are much smaller than the water droplets in clouds, and they are so close together that they scatter all of the sunlight, which is why fog is so dense.
Option C is incorrect because mist is a type of fog that forms when water vapor condenses in the air near the ground. The water droplets in mist are even smaller than the water droplets in fog, and they are so close together that they scatter all of the sunlight, which is why mist is so dense.