**Core Concept**
Stress relaxation in denture base resins refers to the phenomenon where a material's stress decreases over time, resulting in a decrease in its elastic modulus. This is a critical property in dentistry, as it affects the fit and longevity of dentures.

**Why the Correct Answer is Right**
Stress relaxation occurs due to the viscoelastic behavior of polymers, such as those used in denture base resins. These materials exhibit time-dependent deformation under constant stress, leading to a decrease in their elastic modulus. This is attributed to the disentanglement of polymer chains and the relaxation of internal stresses within the material.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as stress relaxation is not directly related to the material's hardness or brittleness.
**Option B:** This option is incorrect as stress relaxation is a time-dependent process, not a temperature-dependent one.

**Clinical Pearl / High-Yield Fact**
Stress relaxation is an important consideration in denture design and fabrication, as it affects the material's ability to withstand occlusal forces over time. Materials with high stress relaxation can result in dentures that fit poorly and require frequent adjustments.

**Correct Answer: C.**