Molecular formula from C:H ratio and mass determines branched alkene structure

Steps:

• C:H ratio of 1:2 matches alkene general formula C_nH_{2n}.
• Relative molecular mass of 56 gives 12n + 2n = 14n = 56, so n=4 and formula is C_4H_8.
• C_4H_8 isomers include straight-chain and branched alkenes; branched specifies 2-methylpropene.
• Option B depicts 2-methylpropene, fitting all criteria.

Why B is correct:

• 2-Methylpropene is the branched C_4H_8 alkene with formula mass (124 + 18 = 56) matching the given ratio per alkene rule C_nH_{2n}.

Why the others are wrong:

• A: Straight-chain but-1-ene, lacks branching.
• C: Cyclobutane, cyclic not branched.
• D: Butane (C_4H_{10}), wrong ratio (1:2.5) and mass (58).

Final answer: B