Which statement correctly describes why a compound is a pure substance?

A compound is a pure substance because its molecule cannot be broken down into simpler particles by physical means.

Explanation:

A pure substance is one whose composition does not vary, even if the physical conditions in which it is found change. For example, water has a formula that is H2O and is always the same, which indicates that it is made up of molecules in which there are 2 hydrogen atoms and 1 oxygen atom. If that formula changed, it would be another different substance.

A pure substance cannot be broken down into other simpler substances using physical methods.  Therefore, a pure substance has its own or defined characteristic properties.

Within the pure substances 2 types are distinguished: Elements and compounds.

nucleus

would be most reasonable answer tbh