The correct order of decreasing acidity of the following aliphatic acids is

The correct option is HCOOH > CH3COOH > (CH3)2CHCOOH > (CH3)3CCOOH.

To understand the order of acidity of the given carboxylic acids, we need to analyze the factors that stabilize their conjugate bases (carboxylate anions). The general formula for the dissociation of a carboxylic acid is:

$R-\mathrm{COOH}\rightleftharpoons R-{\mathrm{COO}}^{-}+H^{+}$

The acidity of a carboxylic acid depends directly on the stability of the carboxylate anion ($R-{\mathrm{COO}}^{-}$):
1. Any factor that stabilizes the negative charge on the carboxylate ion increases the acidity.
2. Any factor that destabilizes the negative charge decreases the acidity.

The alkyl groups attached to the carboxylic acid group act as electron-donating groups due to the inductive effect ($+I$ effect). The $+I$ effect pushes electron density towards the carboxylate group, intensifying the negative charge on the oxygen atoms and destabilizing the carboxylate anion.

The strength of the electron-donating inductive effect ($+I$ effect) of the groups attached to the $-\mathrm{COOH}$ group increases in the following order:

$-H<-{\mathrm{CH}}_3<-\mathrm{CH}{\left({\mathrm{CH}}_3\right)}_2<-C{\left({\mathrm{CH}}_3\right)}_3$

As the +I effect of the alkyl group increases, the stability of the conjugate base decreases, which in turn decreases the acid strength. Therefore:
- Formic acid ($\mathrm{HCOOH}$) has only a hydrogen atom attached, resulting in no $+I$ destabilization. It is the most acidic.
- Acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$) has one methyl group contributing a weak $+I$ effect.
- Isobutyric acid (${\left({\mathrm{CH}}_3\right)}_2\mathrm{CHCOOH}$) has an isopropyl group contributing a stronger $+I$ effect.
- Pivalic acid (${\left({\mathrm{CH}}_3\right)}_3\mathrm{CCOOH}$) has a tert-butyl group contributing the strongest $+I$ effect. It is the least acidic.

Thus, the decreasing order of acidity is:

$\mathrm{HCOOH}>{\mathrm{CH}}_3\mathrm{COOH}>{\left({\mathrm{CH}}_3\right)}_2\mathrm{CHCOOH}>{\left({\mathrm{CH}}_3\right)}_3\mathrm{CCOOH}$