The correct order of increasing acid strength of the compounds is (a) $$CH_3CO_2H$$ (b) $$MeOCH_2CO_2H$$ (c) $$CF_3CO_2H$$ 

Inductive Effect ($$\pm I$$)

The acid strength of a carboxylic acid depends on the stability of its conjugate base (carboxylate anion):

• Electron-Withdrawing Groups ($$-I$$ effect): Disperse the negative charge of the carboxylate ion, stabilizing it and increasing acidity.
• Electron-Donating Groups ($$+I$$ effect): Intensify the negative charge on the carboxylate ion, destabilizing it and decreasing acidity.

Component Analysis:

1. $$\text{CH}_3\text{CO}_2\text{H}$$ (Acetic acid): The methyl group ($-\text{CH}_3$) exerts an electron-donating ($+I$) effect, which destabilizes the conjugate base. It is a weak acid.
2. $$\text{MeOCH}_2\text{CO}_2\text{H}$$ (Methoxyacetic acid): The methoxy group ($-\text{OMe}$) is electronegative and exerts an electron-withdrawing ($-I$) effect through the $-\text{CH}_2-$ spacer, increasing its acidity over acetic acid.
3. $$\text{CF}_3\text{CO}_2\text{H}$$ (Trifluoroacetic acid): The three highly electronegative fluorine atoms exert a powerful, combined electron-withdrawing ($-I$) effect. This strongly stabilizes the anion, making it an exceptionally strong organic acid.
4. $$\text{(CH}_3)_2\text{CHCO}_2\text{H}$$ (Isobutyric acid): The isopropyl group contains two electron-donating methyl groups attached to the $\alpha$-carbon. This creates a stronger $+I$ effect than a single methyl group, making it the weakest acid in the group.

Conclusion:

Arranging the compounds from weakest to strongest acid gives the increasing order:

$$\text{Isobutyric acid (d)} < \text{Acetic acid (a)} < \text{Methoxyacetic acid (b)} < \text{Trifluoroacetic acid (c)}$$

$$\mathbf{d < a < b < c}$$

Answer: Option C — d < a < b < c