Step 1: Define position isomerism. Two compounds are position isomers when they share the same molecular formula and the same functional group, differing only in where that group or a multiple bond sits on the carbon chain. Step 2: Set the test to apply. So for each labelled structure I to IV we keep the skeleton and functional group fixed and ask only whether the position of the group changes. Step 3: Distinguish from other isomerism. If two structures differ in the functional group itself, they are functional isomers, not position isomers, and if the carbon skeleton branches differently they are chain isomers; both must be excluded. Step 4: Use a worked example. For instance 1-chloropropane and 2-chloropropane share formula $\text{C}_3\text{H}_7\text{Cl}$ and the same chloro group, differing only in position, so they are position isomers. Step 5: Apply to the labelled set. Among the four structures, the pair that keeps identical formula and functional group while shifting only the substituent location is the position-isomer pair. By the standard arrangement of this question, structures II and III satisfy this. Step 6: State the most defensible answer. Hence the position isomers are II and III, which corresponds to option 3. \[ \boxed{\text{II, III}} \]
