Final answer:
Carbon monoxide (CO) is the most abundant molecule in interstellar molecular clouds and is used by astronomers to study giant molecular clouds.
Step-by-step explanation:
Carbon monoxide (CO) is the most abundant molecule in an interstellar molecular cloud. It is particularly abundant in interstellar space and is the primary tool that astronomers use to study giant molecular clouds. Carbon monoxide tends to be present wherever molecular hydrogen (H₂) is found in interstellar clouds, making it a good tracer for molecular hydrogen.