Ethanol and dimethyl ether have different chemical and physical properties because they have different
(1) functional groups
(2) molecular masses
(3) numbers of covalent bonds
(4) percent compositions by mass

(1) functional groups

Explanation:

The compounds having similar molecular formula but different arrangement of atoms or groups in space are called isomers and the phenomenon is called as isomerism.  

Functional groups are specific group of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

Dimethyl ether and ethanol are functional isomers which have same molecular formula but different functional groups attached and thus have different physical and chemical properties.

They have same number of atoms and thus have similar molecular mass of 46 and same percent composition by mass. The number of covalent bonds are 8 in both compounds.

a is the correct answer