**Core Concept**
Actinomyces species are gram-positive, facultatively anaerobic bacteria that play a significant role in oral infections, particularly in the formation of periodontal pockets. Their ability to adhere to tooth surfaces is a critical factor in the development of these infections.

**Why the Correct Answer is Right**
The attachment of Actinomyces species to tooth surfaces is facilitated by fimbriae, which are short, hair-like appendages on the bacterial surface. These fimbriae interact with the tooth surface, allowing the bacteria to anchor themselves firmly in place. This attachment is essential for the bacteria to colonize the tooth surface and initiate the infection process. The specific interaction between Actinomyces fimbriae and the tooth surface is mediated by the bacterial adhesin, which recognizes and binds to specific receptors on the tooth enamel.

**Why Each Wrong Option is Incorrect**
**Option B:** Cilia are microtubule-based structures found in eukaryotic cells, such as those in the respiratory epithelium, and are not associated with bacterial attachment to tooth surfaces.
**Option C:** Flagella are long, whip-like appendages found in some bacteria, which are involved in motility rather than attachment.
**Option D:** Capsules are thick layers of polysaccharides or other molecules surrounding some bacteria, which provide protection against environmental stresses but are not involved in attachment to tooth surfaces.

**Clinical Pearl / High-Yield Fact**
Actinomyces species are commonly found in the oral cavity and can be isolated from the teeth, tongue, and other oral tissues. Their ability to adhere to tooth surfaces is a critical factor in the development of periodontal infections, which can lead to the formation of periodontal pockets and the loss of teeth.

✓ Correct Answer: A. Fimbriae.