Isomers are two or more compounds having the same molecular formula but different structural formula.

Among the options given, only propanal and propanone have the same number of C atoms, Hydrogen atoms and Oxygen atoms even though they have different structural formulas, their molecular formulas are the same, hence they are Isomers.

 Propanal - CH\(_3\)CH\(_2\)CHO ⇒ C\(_3\)H\(_6\)O

Propanone - CH\(_3\)COCH\(_3\) ⇒ C\(_3\)H\(_6\)O