A. 2-Methylpropene and but-1-ene
Both have same molecular formula (C₄H₈) but different carbon skeleton. Hence, they are chain isomers. A → III.
B. Cis-but-2-ene and trans-but-2-ene
They differ in spatial arrangement around double bond. Hence, stereoisomers (geometrical isomers). B → I.
C. 2-Butanol and diethyl ether
Both have molecular formula C₄H₁₀O but different functional groups (alcohol and ether). C → IV.
D. But-1-ene and but-2-ene
Position of double bond differs. Hence, position isomers. D → II.
Final Matching: A-III, B-I, C-IV, D-II
Correct Option: A
Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.