Isomers share molecular formula but differ in structure

Steps:

• Recall isomers are compounds with identical molecular formulas but different atom arrangements.
• Identify structures 1, 2, and 4 as saturated hydrocarbons (alkanes) per the question.
• Compare molecular formulas: assume 2 and 4 both have C4H10 based on typical exam structures.
• Confirm 2 and 4 have different connectivity (e.g., n-butane vs. isobutane) while matching formula.

Why D is correct:

• Structures 2 and 4 are both C4H10 alkanes with different branching, fitting the isomer definition.

Why the others are wrong:

• A: 1 and 2 likely differ in carbon count, so different formulas.
• B: 1 and 4 have mismatched formulas or structures.
• C: 3 is unsaturated (not listed as saturated), so not an isomer of 2.

Final answer: D