Given the IUPAC name is pentan-1-oic acid.
Since the root word is pent, there will be five carbons in the parent chain.
The primary suffix is -an-, all the carbon atoms linked with only single bonds in the main carbon chain and the secondary suffix is -oic acid, so the funtional group is carboxylic acid (-COOH).
The position of the funtional group is on the first carbon (-1-oic acid).
Therefore the structural formula is CH3CH2CH2CH2COOH.