I = ClCH₂COOH, II = formic acid, III = CHCl₂COOH, IV = C₆H₅COOH.  Correct acidic strength order is 

EWG increase the acidic strength of aliphatic carboxylic acids by-I effect.

$—\mathrm{I}$ effect of $—{\mathrm{CF}}_3>—{\mathrm{NO}}_2>—\mathrm{CN}>—\mathrm{F}>—\mathrm{Cl}>—\mathrm{Br}>—\mathrm{I}>—\mathrm{Ph}$

EDG decrease the acidic strength of aliphatic carboxylic acids by +I effect

Alkyl groups shows +I effect;

+I effect increase with increase in chain length and bulkiness of alkyl group.

$\therefore$ Acidic strength of ${\text{O}}_2\mathrm{N}-{\mathrm{CH}}_2-\mathrm{COOH}>\mathrm{NC}-{\mathrm{CH}}_2-\mathrm{COOH}>\mathrm{F}-{\mathrm{CH}}_2-\mathrm{COOH}>{\mathrm{CH}}_3-\mathrm{COOH}>\mathrm{R}-{\mathrm{CH}}_2-\mathrm{COOH}$

Inductive effect weakens as the distance increases.

Presence of EWG at $\mathrm{\alpha }$-position makes acetic acid (or) propionic more acidic than formic acid.

Let an EWG is present at $\mathrm{\beta }$-position of acetic acid (or) propionic acid. It is less acidic than formic acid. 

Thus ${\mathrm{Cl}}_2\mathrm{CHCOOH}>{\mathrm{ClCH}}_2\mathrm{COOH}>\mathrm{HCOOH}>{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}$