**Core Concept**
The question is testing the knowledge of a specific stain used in the diagnosis of a premalignant lesion of the lip. This requires an understanding of the histopathological features of oral lesions and the role of staining techniques in their identification.

**Why the Correct Answer is Right**
The correct answer is **Orcein**. Orcein is a histological stain used to diagnose actinic cheilitis, a premalignant lesion of the lip. Actinic cheilitis is a condition characterized by the atrophy of the epithelial layer of the lip, leading to an increased risk of developing squamous cell carcinoma. Orcein staining highlights the presence of elastic fibers, which are often disrupted in actinic cheilitis, allowing for its diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Haematoxylin and Eosin (H&E) is a general-purpose stain used in histopathology, but it is not specific for diagnosing premalignant lesions of the lip.
* **Option B:** Periodic Acid-Schiff (PAS) is a stain used to diagnose conditions such as papillary carcinoma of the thyroid and certain types of fungal infections, but it is not used for diagnosing actinic cheilitis.
* **Option C:** Alcian blue is a stain used to diagnose conditions such as Barrett's esophagus and certain types of gastrointestinal tumors, but it is not used for diagnosing actinic cheilitis.

**Clinical Pearl / High-Yield Fact**
Orcein staining is a specific and sensitive method for diagnosing actinic cheilitis, and its use can help prevent the progression of this premalignant lesion to squamous cell carcinoma.

**Correct Answer: C. Orcein**