**Core Concept**
The underlying principle being tested is the relationship between smoking and **periodontal disease**, which is a chronic inflammatory disease affecting the **periodontium**, the tissues surrounding and supporting the teeth. Smoking is a significant risk factor for periodontal disease due to its effects on the immune system and oral health.

**Why the Correct Answer is Right**
Smoking is known to increase the incidence and severity of periodontal disease by reducing blood flow to the gums, increasing the production of **reactive oxygen species**, and altering the immune response, making smokers more susceptible to **periodontal pathogens**. This leads to a higher likelihood of developing periodontal disease and a worse prognosis for treatment outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the relationship between smoking and periodontal disease is well-established, with smoking being a significant risk factor.
**Option B:** This option is incorrect as it underestimates the impact of smoking on periodontal health.
**Option D:** This option is incorrect because it suggests a decrease in incidence, which contradicts the established knowledge on the effects of smoking on periodontal disease.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that smoking cessation can significantly improve periodontal health outcomes, emphasizing the importance of counseling patients about quitting smoking as part of their dental care plan.

**Correct Answer:** D. Increased.