Two substances have different physical and chemical properties. both substances have
molecules that contain two carbon atoms, one oxygen atom, and six hydrogen atoms. these two substances must be
(1) isomers of each other
(2) isotopes of each other
(3) the same compound
(4) the same hydrocarbon

Option (1) is the correct answer.

Explanation:

When two or more compounds represents same chemical formula but different arrangement of atoms in their structure are known as isomers.

Also, due to different arrangement of atoms the molecules will show different physical and chemical properties.

Isotopes are the atoms which contain same number of protons but they have different number of neutrons. So,they exhibit same chemical properties due to same number of electrons or protons.

Thus,we can conclude that in the given situation the two substances must be isomers of each other.