Final answer:
The part of a nebula that contains the most matter is the giant molecular cloud.
Step-by-step explanation:
The part of a nebula that contains the most matter is the giant molecular cloud. These clouds are the most massive reservoirs of interstellar matter and are the birthplaces of most stars in our galaxy. Giant molecular clouds have cold interiors with temperatures of only 10-20 K, and most of their gas atoms are bound into molecules.