IUPAC name of Isobutyraldehyde is 

$\stackrel{3}{\mathrm{C}}{\mathrm{H}}_3-\underset{\underset{{\mathrm{CH}}_3}{|}}{\stackrel{2}{\mathrm{C}}\mathrm{H}}-\stackrel{1}{\mathrm{C}}\mathrm{HO}$

Number of carbon atoms present in the straight carbon chain gives name to the parent chain which is 3 in this case (Propane). Aldehyde ends with the suffix "al". Since in the structure, the 2nd carbon atom from$CHO$ group has a Methyl group attached to it, so it is called 2-Methylpropanal 

Hence, Correct option is option (B) Methylpropanal.