**Core Concept**
Smooth surface caries is a type of dental caries characterized by the demineralization of enamel on smooth surfaces of teeth, such as the labial or lingual surfaces of anterior teeth. This condition is primarily caused by the accumulation of acid-producing bacteria on these surfaces.

**Why the Correct Answer is Right**
The primary organism involved in smooth surface caries is **Streptococcus mutans**. This bacteria adheres to tooth enamel and produces lactic acid as a byproduct of glucose fermentation, leading to acid production and enamel demineralization. Streptococcus mutans possesses glucosyltransferases, enzymes that facilitate the formation of insoluble glucans from glucose, which in turn contribute to the formation of dental plaque.

**Why Each Wrong Option is Incorrect**
**Option A:** Actinomyces viscosus is more commonly associated with root caries and periodontal disease, rather than smooth surface caries.
**Option B:** Lactobacillus is a genus of bacteria that can contribute to acid production and caries, but is more commonly associated with root caries and not specifically smooth surface caries.
**Option C:** Campylobacter is a genus of bacteria that can cause gastrointestinal infections and is not typically associated with dental caries.

**Clinical Pearl / High-Yield Fact**
Streptococcus mutans is a key organism in the development of dental caries, and its presence and activity can be reduced through good oral hygiene practices, such as regular brushing and flossing.

**Correct Answer: A. Streptococcus mutans**