The correct sequence of acidic strength of the following aliphatic acids in their decreasing order is: $$CH_3CH_2COOH, CH_3COOH, CH_3CH_2CH_2COOH, HCOOH$$

$$CH_3CH_2COOH$$ (propionic acid), $$CH_3COOH$$ (acetic acid), $$CH_3CH_2CH_2COOH$$ (butyric acid), $$HCOOH$$ (formic acid).

Recall the factor affecting acidity

The acidic strength of carboxylic acids depends on the stability of the conjugate base ($$RCOO^-$$). Alkyl groups exhibit a +I (electron-donating inductive) effect, which destabilises the carboxylate anion by pushing electron density towards the negatively charged carboxylate group. More alkyl groups (and longer chains) lead to greater +I effect and thus weaker acidity.

Compare the +I effect

$$HCOOH$$: No alkyl group attached — strongest acid in this series.

$$CH_3COOH$$: One methyl group — moderate +I effect.

$$CH_3CH_2COOH$$: Ethyl group — stronger +I effect than methyl.

$$CH_3CH_2CH_2COOH$$: Propyl group — strongest +I effect, weakest acid.

Arrange in decreasing order of acidic strength

$$HCOOH > CH_3COOH > CH_3CH_2COOH > CH_3CH_2CH_2COOH$$

This matches Option 3.