See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

**Understanding Chain Isomerism** Chain isomerism occurs when compounds with the same molecular formula have different carbon chain arrangements (straight chain vs. branched chain). **Analysis by Carbon Number:** - **2 carbons ($C_2$):** Only one possible structure: $CH_3-CH_3$ (ethane). No isomers possible. - **3 carbons ($C_3$):** Only one possible structure: $CH_3-CH_2-CH_3$ (propane). Cannot form branched isomers with only 3 carbons. - **4 carbons ($C_4$):** Two possible isomers exist: - Straight chain: $CH_3-CH_2-CH_2-CH_3$ (butane) - Branched: $CH_3-CH(CH_3)-CH_3$ (2-methylpropane/isobutane) **Conclusion:** Chain isomerism first appears at **4 carbon atoms**. With fewer carbons, the carbon skeleton cannot be arranged in different ways while maintaining the same molecular formula. **Answer: (4) 4**