Branched-chain isomer of butane has a carbon branch

Steps:

• Recall butane's formula: C4H10, with straight-chain n-butane (CH3-CH2-CH2-CH3) and branched isobutane (2-methylpropane, (CH3)2CH-CH3).
• Identify branched isomer: features a central carbon attached to three groups, not a linear chain.
• Examine diagrams: look for non-linear structure with four carbons and ten hydrogens.
• Match to isobutane: branched chain shows one carbon bonded to three others.

Why A is correct:

• Diagram A depicts 2-methylpropane, where a CH group connects to three CH3 groups, matching the branched isomer definition for C4H10.

Why the others are wrong:

• B shows straight-chain n-butane (linear CH3-CH2-CH2-CH3).
• C represents an invalid structure (e.g., ring or extra bond, not C4H10).
• D illustrates pentane or unrelated chain (five carbons).

Final answer: A