1. Idea involved

For a dicarboxylic acid to form an anhydride easily in the presence of a dehydrating agent, the two carboxyl groups should be able to come close and undergo intramolecular dehydration.

This is most favorable when a 5-membered or 6-membered cyclic anhydride can form.

2. General trend

If the dicarboxylic acid is of the type $\mathrm{HOOC-(CH_2)_n-COOH}$ then on dehydration it forms a cyclic anhydride.

• For $n=1$ (malonic acid), a 4-membered cyclic anhydride would form → highly strained, so difficult.
• For $n=2$ (succinic acid), a 5-membered cyclic anhydride forms → favorable.
• For $n=3$ (glutaric acid), a 6-membered cyclic anhydride forms → favorable.
• For $n=4$ (adipic acid), a 7-membered cyclic anhydride would form → less favorable than 5- or 6-membered rings.

Among the commonly considered dicarboxylic acids, the least reactive for anhydride formation is the one that would need to form the most unfavorable ring.

3. Most unfavorable case

A 4-membered cyclic anhydride is much more strained than 5- or 6-membered cyclic anhydrides. Therefore, malonic acid is least reactive toward forming an anhydride.

4. Conclusion

The correct choice is malonic acid, which corresponds to Option A.

Thus, the stored answer is consistent.