**Core Concept**
Lichenisation is a process of skin thickening characterized by an increase in the number of collagen fibers, hypertrophy of the epidermis, and hyperplasia of the dermal papillae. This response is typically seen in chronic dermatitis, eczema, and other inflammatory skin conditions.

**Why the Correct Answer is Right**
Lichenisation occurs primarily in the _stratum spinosum_ of the epidermis, where the epidermal cells undergo hyperplasia and the intercellular connections are strengthened. This thickening of the epidermis leads to a characteristic lichenoid appearance. The process is mediated by various inflammatory cytokines and growth factors that stimulate collagen synthesis and epidermal cell proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** The _stratum basale_ is the deepest layer of the epidermis, where epidermal stem cells reside, but it is not primarily involved in lichenisation.

**Option B:** The _stratum corneum_ is the outermost layer of the epidermis, composed of dead keratinocytes, and is not directly involved in the thickening process.

**Option C:** The dermal layer, particularly the _papillary dermis_, does undergo changes in lichenisation, but the primary response occurs in the epidermal layer.

**Clinical Pearl / High-Yield Fact**
The term "lichenisation" is often used interchangeably with "lichenoid dermatitis," a type of skin reaction that mimics lichen planus, characterized by pruritic, polygonal, flat-topped papules.

**Correct Answer:** C.