The IUPAC name of the following compound is

 

The carboxylic acid group is the functional group while the alcohol is a substituent. The length of the carbon chain is calculated such that the longest chain including the carboxylic group is picked.

 So we have a five-membered carbon chain with the suffix (-oic acid) of pentanoic acid replacing the suffix (-ane) of pentane. The substituents connected receive the lowest number so, the hydroxyl and methyl substituents are assigned numbers 3 and 4, and they are named in alphabetical order  respectively.