This question involves understanding the reactivity of dicarboxylic acids in forming anhydrides with a dehydrating agent. The key concept is ring strain in the resulting cyclic anhydride. Dicarboxylic acids can form cyclic anhydrides if the two carboxylic acid groups are positioned such that they can form a 5- or 6-membered ring, which are stable due to minimal ring strain. Acids that would form rings larger or smaller than these experience higher strain, making anhydride formation less favorable.

Let's analyze the given structures (though images are provided, I'll describe based on common knowledge):

Option A: Malonic acid (HOOC-CH2-COOH) - forms a 5-membered cyclic anhydride (succinic anhydride analog), low strain, highly reactive.

Option B: Phthalic acid (o-benzenedicarboxylic acid) - forms a 5-membered ring fused with benzene, stable, highly reactive.

Option C: Adipic acid (HOOC-(CH2)4-COOH) - would form a 7-membered ring anhydride, which has significant ring strain, less stable, less reactive.

Option D: Succinic acid (HOOC-(CH2)2-COOH) - forms a 5-membered cyclic anhydride, low strain, highly reactive.

Thus, adipic acid (Option C) is least reactive because it forms a 7-membered ring anhydride with high strain, making dehydration difficult.

Related Topics and Formulae

Ring Strain Theory: Smaller rings (3,4) and larger rings (>6) have higher strain due to angle deviation or transannular interactions. 5- and 6-membered rings are most stable.

Anhydride Formation Reaction: $2R-\mathrm{COOH}\to (R-\mathrm{CO})2O+H_{2}O$ (for intermolecular) or intramolecular for dicarboxylic acids.

Factors Affecting Reactivity: Proximity of -COOH groups (1,2- or 1,3-diacids are reactive; 1,4- or higher are less reactive), steric hindrance, and ring stability.