**Core Concept**
Endodontically treated molars exhibit a reduction in strength and toughness due to alterations in the dentin structure, primarily resulting from changes in collagen cross-linking and dehydration. These changes compromise the dentin's mechanical properties, making it more susceptible to fracture.

**Why the Correct Answer is Right**
The reduction in strength and toughness is attributed to the disruption of collagen cross-linking, which is mediated by the enzyme lysyl oxidase. In endodontically treated molars, the removal of the dentinal tubules' contents and the subsequent dehydration of the dentin lead to the degradation of collagen cross-links. This results in a loss of dentin's mechanical integrity, making it more prone to fracture. The dehydration of dentin also leads to a reduction in its toughness, as the water content plays a crucial role in dissipating stress and absorbing impact.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may seem plausible, but it does not accurately reflect the primary mechanism of reduction in strength and toughness in endodontically treated molars.
* **Option B:** This option is incorrect because the reduction in strength and toughness is not solely attributed to the removal of the pulp tissue. While the pulp tissue does play a role in maintaining the dentin's health, its removal is not the primary cause of the reduction in mechanical properties.
* **Option C:** This option is incorrect because the reduction in strength and toughness is not solely attributed to the dehydration of dentin. While dehydration does play a role, it is not the primary mechanism.

**Clinical Pearl / High-Yield Fact**
When treating endodontically treated molars, it is essential to consider the compromised mechanical properties of the dentin. This can be achieved by using restorative materials with high compressive strength and by minimizing the stress concentrations in the tooth.

**Correct Answer: C. 30%**