The IUPAC name of the crotonaldehyde is

Crotonaldehyde's molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_6\mathrm{O}$. It comes under the category of u Unsaturated aldehydes.  The substance is known by its common name, crotonaldehyde. To find the compound's IUPAC name, follow the IUPAC regulations.

It is written as- ${\mathrm{CH}}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO}$
The crotonaldehyde's IUPAC name is determined by applying the IUPAC rule.
Rule 1: Identify the parent chain, or longest continuous chain, of carbon atoms.  The longest continuous chain in the crotonaldehyde structure has four carbon atoms.
Rule 2: Assign the functional group the lowest priority in the numbering of the chain.  Aldehyde serves as the functional parent chain in crotonaldehyde, thus start counting the parent chain at the carbon atom where the carbonyl group $(\mathrm{C}=\mathrm{O})$ is attached. 
$\stackrel{4}{\mathrm{C}}{\mathrm{H}}_3- \stackrel{3}{\mathrm{C}}\mathrm{H} = \stackrel{2}{\mathrm{C}}\mathrm{H} - \stackrel{1}{\mathrm{C}}\mathrm{HO}$
Because the parent chain has four carbon atoms and an aldehyde functional group, the compound's root name is butanal. However, the complex also has a C = C atom in position 2, giving the crotonaldehyde its IUPAC name of 2-butenal or but-2-en-1-al.

Hence, the correct answer is option C.