**Core Concept**
The question is testing the knowledge of malignant eyelid tumours, specifically the most common type. Malignant eyelid tumours are rare, but it's essential to identify the most common one for diagnosis and treatment purposes.

**Why the Correct Answer is Right**
The most common malignant eyelid tumour is Basal Cell Carcinoma (BCC). BCC arises from the basal cell layer of the skin and is often associated with sun exposure. It typically presents as a slow-growing, painless nodule or ulcer on the eyelid. The growth pattern of BCC is characterized by local invasion into surrounding tissue, but it rarely metastasizes.

**Why Each Wrong Option is Incorrect**
**Option A:** Squamous Cell Carcinoma (SCC) is a type of skin cancer, but it's less common on the eyelid compared to BCC. SCC tends to be more aggressive and has a higher potential for metastasis.
**Option B:** Melanoma is a type of skin cancer that originates from melanocytes, but it's relatively rare on the eyelid. While melanoma can be aggressive, it's not the most common malignant eyelid tumour.
**Option C:** Sebaceous gland carcinoma is a rare type of eyelid tumour that arises from the meibomian glands. While it's a serious condition, it's less common than BCC.

**Clinical Pearl / High-Yield Fact**
Remember the "ABCDs of Eyelid Tumours": Basal Cell Carcinoma (BCC), Sebaceous Gland Carcinoma (SGC), Squamous Cell Carcinoma (SCC), and Melanoma. This will help you to quickly identify the most common malignant eyelid tumours.

**Correct Answer:** C. Squamous Cell Carcinoma