What do we call the substance which is composed of two or more elements, and chemically combined in a fixed ratio by mass?

The correct option is A

Compound

Elements combine to form compounds. A compound called calcium carbonate with the formula ${\text{CaCO}}_3$ has a fixed proportion by mass of calcium, carbon and oxygen (in 10:3:12 ratio). Any proportion that is different from this is no longer called calcium carbonate because the chemical nature of the compound changes.