**Core Concept**
The height of contour (HOC) of a tooth is a critical concept in restorative dentistry, referring to the area on the tooth surface where the curve of the tooth changes direction. It is essential to identify the HOC to ensure proper fitting and retention of dental restorations. The HOC is typically determined by the survey line, a line that marks the point of maximum curvature of the tooth surface.

**Why the Correct Answer is Right**
The survey line is the key to determining the height of contour because it represents the point where the tooth surface transitions from one curvature to another. This line is typically marked on a dental model or a radiograph and serves as a reference point for dentists to plan and execute restorative procedures. The survey line helps to identify the area where the tooth surface is most curved, which is essential for ensuring proper fit and retention of dental restorations.

**Why Each Wrong Option is Incorrect**
**Option B:** Non undercut area is incorrect because it does not accurately represent the height of contour. The non undercut area refers to the region on the tooth surface where there are no undercuts, but it does not necessarily correspond to the point of maximum curvature.

**Option C:** Undercut area is incorrect because it is actually the opposite of the height of contour. Undercuts are areas on the tooth surface where the curvature is more pronounced, creating a space that can trap restorative materials.

**Option D:** None of the above is incorrect because the survey line is indeed the correct answer.

**Clinical Pearl / High-Yield Fact**
Mnemonically, you can remember the survey line as the "S" in "Survey Line" which stands for "Surface" or "Shape" of the tooth. This helps to recall the concept of the height of contour and its significance in restorative dentistry.

✓ Correct Answer: A. Survey line