**Core Concept**
The periodontal pocket is a pathological space formed between the gingiva and the tooth, characterized by the accumulation of various substances and microorganisms. This pocket is a result of the progression of periodontal disease, which leads to inflammation and destruction of the periodontal tissues.

**Why the Correct Answer is Right**
The periodontal pocket is primarily inhabited by microorganisms, such as bacteria, viruses, and fungi, which are responsible for the progression of periodontal disease. These microorganisms produce various virulence factors, including enzymes, toxins, and biofilms, which contribute to tissue destruction and inflammation. Additionally, the periodontal pocket contains the products of microbial metabolism, such as metabolic waste, and debris from the host tissues. This accumulation of microorganisms and their products leads to the characteristic signs and symptoms of periodontal disease, including pocket formation, bleeding on probing, and loss of attachment.

**Why Each Wrong Option is Incorrect**
**Option A:** Living and degenerated leucocytes are present in the periodontal pocket, but they are not the primary inhabitants. While leucocytes play a crucial role in the host's immune response to periodontal disease, they are not the main components of the periodontal pocket.
**Option C:** Gingival fluids and salivary mucin are present in the periodontal pocket, but they are not the primary contents. These substances contribute to the pocket's environment, but they are not the main drivers of periodontal disease progression.
**Option D:** Desquamated epithelial cells are present in the periodontal pocket, but they are not the primary inhabitants. While these cells are a result of the host's response to periodontal disease, they are not the main components of the pocket.

**Clinical Pearl / High-Yield Fact**
Remember that the presence of microorganisms and their products in the periodontal pocket is a key indicator of periodontal disease. The accumulation of these substances leads to tissue destruction and inflammation, making periodontal therapy crucial for preventing further disease progression.

**✓ Correct Answer: B. Microorganisms, their products and debris**