Isomers share the same molecular formula but differ in structure Steps:

• Determine molecular formulas for structures 1, 2, and 4 by counting C and H atoms.
• Compare formulas: isomers must match exactly for the same number of carbons.
• Examine structural arrangements: check for different carbon chain branching or positioning.
• Confirm saturation: all are alkanes (C_nH_{2n+2}), so focus on isomerism type (e.g., chain or position).

Why A is correct:

• Structures in A (e.g., 1 and 2) have identical C_5H_{12} formula but different chain arrangements, fitting alkane isomer definition.

Why the others are wrong:

• B: Different formulas (e.g., C_4H_{10} vs. C_5H_{12}), so not isomers.
• C: Same formula but identical structures, not different arrangements.
• D: One unsaturated (double bond), violating saturated hydrocarbon rule.

Final answer: A