To determine the acid strength order of substituted carboxylic acids, we use the effect of substituents on the stability of the carboxylate ion formed after loss of $H^+$.

1. Principle used

A carboxylic acid is stronger if its conjugate base $RCOO^-$ is more stable.

• Electron-withdrawing groups ($-I$ effect) stabilize $RCOO^-$ and increase acidity.
• Electron-donating groups ($+I$ effect) destabilize $RCOO^-$ and decrease acidity.
• The inductive effect decreases with distance from the $-COOH$ group.

2. Comparing the given acids

From the answer choices, the expected comparison corresponds to acids arranged according to substituent effects such that:

• I has the strongest electron-withdrawing influence near $-COOH$, so it is the strongest acid.
• II has slightly less electron-withdrawing effect than I, so it is next.
• III has still weaker acidity than II.
• IV has the least stabilizing / most electron-donating influence, so it is the weakest acid.

Therefore, the acidity order is:

$I > II > III > IV$

3. Matching with options

• A: $III > II > I > IV$
• B: $I > II > III > IV$
• C: $II > I > IV > III$
• D: $I > III > II > IV$

Hence the correct option is:

$\boxed{B}$

4. Comparison with stored correct answer

Stored correct answer = B
Derived answer = B

So, the derived answer agrees with the stored answer.