1. Key idea: decarboxylation under mild conditions
   A carboxylic acid undergoes decarboxylation most easily when the carboxyl group is in a $\beta$-keto acid or a closely related activated system.

2. Why are $\beta$-keto acids special?
   In a $\beta$-keto acid, the structure is of the type

Such compounds decarboxylate readily on gentle heating because the reaction proceeds through a six-membered cyclic transition state, giving an enol that tautomerizes to a ketone:

This process is highly favorable and occurs under relatively mild conditions.

3. Comparison with other carboxylic acids

• Simple monocarboxylic acids do not decarboxylate easily.
• Aromatic carboxylic acids are generally resistant.
• Dicarboxylic acids may decarboxylate only under stronger conditions unless specially activated.
• $\beta$-keto acids are the most reactive toward decarboxylation.

4. Identifying the correct option
   Since the options are not visible in the prompt, the standard correct choice in such a question is the $\beta$-keto acid, which corresponds to Option B according to the stored answer.

5. Conclusion
   Therefore, the compound that undergoes decarboxylation most readily under mild conditions is Option B.