**Core Concept**
The line angle is a crucial concept in cavity preparation, referring to the 90-degree internal angle formed by the intersection of two walls of a cavity preparation. This angle is essential for the proper retention and resistance form of the restoration.

**Why the Correct Answer is Right**
The line angle is a critical factor in ensuring the structural integrity and longevity of the restoration. When the line angle is 90 degrees, it allows for the optimal distribution of stresses and forces across the restoration, thereby reducing the risk of fracture or marginal breakdown. This is achieved through the use of a converging or diverging wall configuration, which helps to dissipate occlusal forces and prevent the restoration from being subjected to excessive stress.

**Why Each Wrong Option is Incorrect**
**Option A:** A line angle of 45 degrees is not sufficient to provide adequate retention and resistance form, as it does not allow for the optimal distribution of stresses across the restoration.
**Option B:** A line angle of 135 degrees is excessive and can lead to an increase in the risk of marginal breakdown and restoration fracture.
**Option C:** A line angle of 0 degrees is not a valid option, as it would result in a flat surface with no defined angle.

**Clinical Pearl / High-Yield Fact**
When preparing a cavity, it is essential to maintain a 90-degree line angle to ensure the proper retention and resistance form of the restoration. This can be achieved through the use of a converging or diverging wall configuration, which helps to dissipate occlusal forces and prevent the restoration from being subjected to excessive stress.

**Correct Answer:** C. 90 degrees