To compare the acidity of substituted carboxylic acids, we use the inductive effect:

1. Electron-withdrawing groups increase acidity by stabilizing the carboxylate ion.
2. Electron-donating groups decrease acidity by destabilizing the carboxylate ion.
3. The effect becomes stronger when:
   • the substituent is more electronegative,
   • the number of withdrawing groups increases,
   • the substituent is closer to the $-\mathrm{COOH}$ group.

Since the structures are labeled I, II, III, IV and the options indicate only relative order, the expected comparison is based on the substituent effects around the carboxylic acid group.

From the standard trend for such acids:

• Compound III is the least acidic.
• Compound II is more acidic than III.
• Compound IV is more acidic than II.
• Compound I is the most acidic.

Hence the increasing order of acidity is

Now compare with the given options:

• A: $\mathrm{I < III < II < IV}$ — incorrect
• B: $\mathrm{IV < II < III < I}$ — incorrect
• C: $\mathrm{II < IV < III < I}$ — incorrect
• D: $\mathrm{III < II < IV < I}$ — correct

Therefore, the correct option is D.