In which scenario would you expect to find a positional isomer?

Positional isomers are compounds that have the same molecular formula but differ in the arrangement of atoms, particularly the position of functional groups or substituents on a carbon skeleton. This type of isomerism arises because the changes in the position of the functional group can lead to variations in the compound's properties and reactivity, even though the overall number of atoms and their types remain unchanged.

In this context, the scenario describes compounds that share the same molecular formula but exhibit different arrangements of functional groups. For example, consider butanol (which can exist as n-butanol and isobutanol). Both have the same molecular formula ( C_4H_{10}O ) but differ in the placement of the hydroxyl (-OH) group. This demonstrates the concept of positional isomerism effectively, as the location of the functional group is critical to defining the identity and properties of the compound.

The other scenarios would not lead to positional isomers. Different molecular formulas indicate completely different compounds rather than variations of the same compound. Variants with different functional groups, while they could potentially have isomers, are not specifically positional isomers as they involve a different type of functional group rather than a mere shift in position. Lastly, the cyclic