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

The correct order of decreasing acidity of the given aliphatic acids is:
HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH

Explanation:
The acidity of carboxylic acids is determined by the stability of the carboxylate anion formed after the loss of a proton (H⁺). Factors that stabilize the conjugate base (carboxylate ion) increase the acidity of the parent acid, whereas factors that destabilize it decrease the acidity.

1. Inductive Effect (+I Effect):
Alkyl groups are electron-donating groups due to the positive inductive effect (+I effect). When an electron-donating group is attached to the carboxyl carbon, it increases the electron density on the carboxylate group. This destabilizes the negative charge of the conjugate base, making it less stable and thereby decreasing the acidity of the carboxylic acid.

2. Analysis of the given acids:
Let us analyze the inductive effect of the groups attached to the carboxyl group (-COOH) in each of the given acids:

• HCOOH (Formic acid): The hydrogen atom (H-) has negligible inductive effect. There is no electron-donating alkyl group to destabilize the carboxylate anion (HCOO^-). Therefore, formic acid is the most acidic among the group.

• CH₃COOH (Acetic acid): It contains one methyl group (-CH₃), which exerts a +I effect, destabilizing the acetate ion (CH₃COO^-) compared to the formate ion.

• (CH₃)₂CHCOOH (Isobutyric acid): It contains an isopropyl group, which has two electron-donating methyl groups attached to the α-carbon. This exerts a stronger +I effect than a single methyl group, further decreasing its acidity.

• (CH₃)₃CCOOH (Trimethylacetic acid / Pivalic acid): It contains a tert-butyl group, which has three electron-donating methyl groups attached to the α-carbon. This exerts the strongest +I effect among all the given acids, making its conjugate base the least stable and consequently making it the least acidic acid.

Conclusion:
As the number of alkyl groups attached to the carboxyl carbon increases, the electron-donating +I effect increases, which destabilizes the conjugate base and decreases the acidity. Thus, the acidity decreases in the order:
HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH