Answer
The order of acidity is :
$CH_3CH_2COOH \lt CH_3CHClCOOH \lt CH_3CHCNCOOH \lt CH_3CHNO_2COOH$
Work Step by Step
We know, the stronger acid has a more stable conjugate base. In these compounds, the stronger is the electron withdrawing effect of the substituent , the more stable is the carboxylate ion, and the acid is stronger. The electron withdrawing effect of $NO_2$ is stronger than that of $CN$, $Cl$ and $H$. So, The order of acidity is :
$CH_3CH_2COOH \lt CH_3CHClCOOH \lt CH_3CHCNCOOH \lt CH_3CHNO_2COOH$
