What type of isomerism is exhibited by compounds with different functional groups?

Functional group isomerism occurs when compounds have the same molecular formula but possess different functional groups. This type of isomerism highlights the way the arrangement of atoms results in distinct chemical properties and behaviors because different functional groups often lead to different reactivity and characteristics of the compounds. For instance, butanol (an alcohol) and butanal (an aldehyde) both have the formula C4H8O, but their differing functional groups give rise to unique physical and chemical properties.

In contrast, structural isomerism involves variations in how atoms are connected within the molecule, not necessarily limited to just functional groups. Stereoisomerism deals with the spatial arrangement of atoms in molecules with the same connectivity. Geometric isomerism is a specific type of stereoisomerism that refers to the arrangement of groups around a double bond or a ring structure. Thus, functional group isomerism clearly stands out as the correct choice in this context.