$$\textbf{Correct option: D}$$
$$\textbf{Solution:}$$
Option A:
$$\bullet$$ Radiation fog forms after sunset when the ground starts cooling down by infrared thermal radiations.
$$\bullet$$ Usually, radiation fog forms at night and get vanished after sunrise. But, in winter months, it persists all day long, specially in the areas which are surrounded by high ground like a valley.
$$\bullet$$ Valley fog is the radiation fog.
$$\bullet$$ Radiation fog is not more susceptible on hill tops.
Option B:
$$\bullet$$ Summer fog forms when moist air passes over cool surface. It occurs in areas like coastal area of California.
$$\bullet$$ It has been seen that the summer fog decreased by $$33\%$$ in California during 20th century.
$$\bullet$$ This decrease in summer fog is affecting the climate change of California and its affects has been seen on its ecology.
$$\bullet$$ For example, Redwoods are not growing well in the coastal area of California due to this climate change caused due to decrease in summer fog.
Option C:
$$\bullet$$ Fog refers to a visible suspension of fine liquid and solid particles in air near the earth's surface.
$$\bullet$$ It is composed of very tiny water droplets and can have ice crystals.
Option D:
$$\bullet$$ All the given statements are correct.
Hence the correct option is option D.