Answer
The answer is attached below.
Work Step by Step
Aldehydes are named according to the total number of carbon atoms present in the hydrocarbon chain containing the aldehyde group.
In 2-methylbutanal, total 4 carbon atoms are present in the hydrocarbon chain containing the aldehyde group, and carbons are numbered from where the aldehyde group is present. Second carbon from the right contains a methyl group.