She told me that she knows someone who studies the stars

Solution

"Astronomer" is the correct one-word substitute as it refers to a person who studies celestial bodies such as stars and planets. (1) Chemist: A chemist studies chemicals and their reactions, which does not relate to the study of stars and planets. (3) Geologist: A geologist studies the Earth and its processes, including rocks and minerals, not celestial objects. (4) Biologist: A biologist studies living organisms and their interactions with the environment, not celestial bodies.