Acidity of Carboxylic Acids

Acidity of carboxylic acids depends on the stability of the conjugate carboxylate anion ($\text{RCOO}^-$). Electron-donating inductive groups ($+I$) intensify the negative charge on the carboxylate group and destabilize it, thereby reducing acid strength. Formic acid (HCOOH) has no alkyl group and is the strongest acid. Acetic acid has one $+I$ methyl group, isobutyric acid has an isopropyl group (two methyls), and pivalic acid has a tert-butyl group (three methyls), making it the weakest acid. The decreasing order of acidity is $\text{HCOOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} > (\text{CH}_3)_3\text{CCOOH}$.

Electron-withdrawing groups (–I effect) stabilise the carboxylate ion and increase acid strength; electron-releasing groups (+I) decrease it.
F is more electronegative than Cl, so $CF_3COOH > CCl_3COOH$.
The $CH_3$ group in acetic acid has a +I effect, so HCOOH > $CH_3COOH$.
Order: $CF_3COOH$ (B) > $CCl_3COOH$ (A) > HCOOH (D) > $CH_3COOH$ (C)

An electron withdrawing substituent increases the acidity of a carboxylic acid: by its $-I$ (inductive) effect it increases the polarity of the O-H bond and stabilises the carboxylate anion formed.
The halogen atoms are electron withdrawing, so all three haloacetic acids are stronger than acetic acid, in which $CH_3$ is an electron releasing group.
Electronegativity decreases in the order: F > Cl > Br
Hence the $-I$ effect also decreases in the same order.
Therefore the correct order of acidity is: $FCH_2COOH \gt ClCH_2COOH \gt BrCH_2COOH \gt CH_3COOH$
(The $pK_a$ values are about 2.6, 2.9, 2.9 and 4.8 respectively.)