What functional group is present in all alcohols?

The correct option is -OH.

The correct option is -OH. -OH is correct because it directly supports the approved learning objective to recall the first four members of a homologous series of alcohols as methanol, ethanol, propanol and butanol. This belongs to Alcohols within Reactions of alkenes and alcohols (chemistry only), so the answer must use the correct organic chemistry context. The other options are incorrect when they confuse the organic family, formula type, reaction condition, product, or property being tested. Keep molecular formula, structural formula, displayed formula, and general formula distinct. Do not confuse alkanes with alkenes, saturated with unsaturated, cracking with combustion, polymers with monomers, or hydrocarbons with oxygen-containing alcohols and carboxylic acids. When formulae are used, preserve the stored notation exactly and explain the GCSE chemistry idea in words rather than using unsupported displayed-formula diagrams.