IUPAC name of (C2H5)2CHCH2OH(C2H5)2CHCH2OH is
2-Ethylbutan-1-ol
The IUPAC nomenclature is done by determining the main carbon chain which contains the most active functional group. The longest hydrocarbon chain in the given compound consists of five carbon atoms, but it is not the main chain because it does not contain the functional group (-OH). The other carbon chain consists of four carbon atoms with -OH group. The carbon attached to the -OH functional group is labeled as 1.It is a saturated carbon chain, hence the word root with suffix is butanol. An alkyl group attached to the second carbon atom is 'ethyl' group. Therefore, the correct nomenclature is 2-Ethylbutan-1-ol.
The name 3-ethylbutan-4-ol is not possible because the labelling of the carbon atoms is done with the carbon atom having the highest functional group is given the label 1.