**Core Concept**
Polycarboxylate cement is a type of dental cement used for luting crowns, bridges, and orthodontic appliances. Adhesive failure of polycarboxylate cement occurs when the bond between the cement and the tooth structure is disrupted.

**Why the Correct Answer is Right**
Adhesive failure of polycarboxylate cement is most commonly due to the lack of micromechanical interlocking between the cement and the tooth enamel. This is because polycarboxylate cement has a hydrophilic surface that does not effectively engage with the hydrophobic enamel surface. As a result, the cement may not form a strong bond, leading to adhesive failure. The hydrophilic nature of the cement also makes it prone to water sorption, which can further compromise the bond strength.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the adhesive failure of polycarboxylate cement.
**Option B:** This option is not a common cause of adhesive failure of polycarboxylate cement.
**Option C:** This option is a possible cause of mechanical failure of polycarboxylate cement, but not adhesive failure.

**Clinical Pearl / High-Yield Fact**
Polycarboxylate cement is a pH-sensitive cement that releases fluoride ions, which can help prevent secondary caries. However, its adhesive properties are limited, making it less suitable for luting metal restorations.

**Correct Answer: C.**