The words, "in the sky" in the passage indicate that the reference in the passage is to men who studied astronomy. Astronomy is the branch of science which deals with celestial objects, space and the physical universe as a whole.
The word, "weather" refers to the state of the atmosphere at a particular place and time, especially with respect to factors like heat, cloudiness, humidity, etc. The word, "climate" refers to the weather conditions prevailing in an area in general or over a long period of time. The word, "environment" means: The surroundings or conditions in which a person, animal or plant operate in.All these three words have no connection with astronomy. Hence, options A, C, and D are incorrect.
Option B is most appropriate as "stars" are celestial bodies, which are visible in the sky and were studied by astronomers.