**Core Concept**
The question is about the composition of dental calculus, which is a hardened deposit on teeth. Dental calculus is primarily composed of **mineralized** plaque, with various crystal forms of calcium phosphate being key components.

**Why the Correct Answer is Right**
The correct answer, **D. Octacalcium phosphate**, is less commonly found in dental calculus compared to other forms. This is because **hydroxyapatite** and other calcium phosphate crystals are more stable and predominant in the calculus.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydroxyapatite is a major component of dental calculus.
**Option B:** Dicalcium phosphate dihydrate is also found in calculus, though less than hydroxyapatite.
**Option C:** Tricalcium phosphate can be present in dental calculus as well.

**Clinical Pearl / High-Yield Fact**
Dental calculus can lead to **periodontal disease**, making its prevention and removal important for oral health.

**Correct Answer:** D. Octacalcium phosphate