9. When two compounds are made up of the same number and kind of atoms, but differ in
their molecular structure they are known as:
A) isotopes
B) homologs
C) isomers
D) hydrocarbon

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.