Geraniol and nerol show cis-trans isomerism around a double bond

Steps:

• Identify molecular formulas: Both C₁₀H₁₈O, confirming constitutional isomerism is not chain or positional.
• Examine structures: Both are unsaturated alcohols with a double bond between carbons 2-3 and 6-7 in a 10-carbon chain.
• Note configuration difference: Geraniol has trans configuration at the 2-3 double bond; nerol has cis.
• Classify: This restricted rotation around the C=C bond defines geometrical (cis/trans) isomerism.

Why B is correct:

• Geometrical isomerism arises from cis-trans arrangements due to double bond rigidity, as in alkenes where substituents differ on each carbon (definition from organic chemistry stereoisomerism).

Why the others are wrong:

• A: Chain isomerism requires different carbon skeletons, but both share the same branched chain.
• C: Optical isomerism needs chiral centers with non-superimposable mirror images, absent here.
• D: Positional isomerism involves different functional group locations, but OH and double bonds are identically placed.

Final answer: B