Answer
The IUPAC name of the compound is $3-methylbut-2-enoic \hspace1mm acid$
Work Step by Step
The acid has 4 carbon atoms in its longest carbon chain containing the carboxylic functional group. It has a double substituent at position 2, and a $methyl$ substituent at position 3. So, the IUPAC name of the compound is $3-methylbut-2-enoic \hspace1mm acid$