**Core Concept**
Actinomycosis is a chronic bacterial infection caused by Actinomyces species, typically affecting the face, neck, and cervico-facial region. The primary mode of entry is through mucosal breaches, often following dental procedures or periodontal disease.

**Why the Correct Answer is Right**
Actinomycosis tends to affect the facio-cervical region due to the presence of Actinomyces in the oral cavity. The bacteria thrive in anaerobic environments, often colonizing the gingival crevice or dental plaque. When the mucous membrane is compromised, Actinomyces can invade the surrounding tissues, leading to the characteristic chronic infection with abscess formation and draining sinuses.

**Why Each Wrong Option is Incorrect**
**Option A:** The thorax is not a common site for actinomycosis, although it can occur in immunocompromised patients or those with a history of thoracic surgery.
**Option C:** Actinomycosis is extremely rare in the liver, as the bacteria typically infect mucosal surfaces rather than the liver parenchyma.
**Option D:** While actinomycosis can affect the pelvis, especially in patients with intrauterine devices or pelvic surgery, it is less common than facio-cervical involvement.

**Clinical Pearl / High-Yield Fact**
Actinomycosis is often mistaken for other chronic infections, such as tuberculosis or sarcoidosis, due to its similar clinical presentation. However, the presence of characteristic "sulfur granules" in the pus or tissue samples is a key diagnostic feature.

**✓ Correct Answer: B. Facio - cervical region**