Answer:
a) visible masses of particles in the air.
Step-by-step explanation:
Although the passage is not included in the question, it is still possible for us to answer this question based on the options provided. In this question, you are asking for the definition of the word "clouds." In order to provide a definition of a word, we need to focus on what the word explains, or the idea that it wants to convey. The definition should ultimately tell us what an idea "is." In this example, option B describes the word "clouds." Moreover, option C tells us how the clouds react and option D tells us what the effect of the clouds is. However, option A is the only that explains the meaning of the word.