**Core Concept**
The question is testing the knowledge of the most common eyelid tumor, which is a fundamental aspect of ophthalmology and dermatology. Basal cell carcinoma (BCC) is a type of skin cancer that originates from the basal cell layer of the epidermis.

**Why the Correct Answer is Right**
Basal cell carcinoma (BCC) is the most common eyelid tumor, accounting for approximately 85% of all eyelid cancers. This is due to its propensity to occur on sun-exposed areas, such as the eyelids. BCC arises from the basal cell layer of the epithelium, where it forms a well-differentiated tumor that can invade surrounding tissue but rarely metastasizes. The pathogenesis of BCC involves mutations in the PTCH1 gene, leading to activation of the Hedgehog signaling pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** Sebaceous carcinoma is a rare type of eyelid cancer, accounting for only 1-2% of all eyelid tumors. It originates from the sebaceous glands and is more aggressive than BCC.
**Option C:** Squamous cell carcinoma (SCC) is another type of skin cancer that can occur on the eyelid, but it is less common than BCC. SCC is more aggressive than BCC and has a higher potential for metastasis.
**Option D:** Malignant melanoma is a type of skin cancer that originates from melanocytes, but it is relatively rare on the eyelid. Melanoma is more aggressive than BCC and has a higher potential for metastasis.

**Clinical Pearl / High-Yield Fact**
The "Nevus sebaceous" is a benign hamartoma of the sebaceous glands that can undergo malignant transformation to sebaceous carcinoma, especially after puberty. This is a classic exam trap, and students should be aware of this correlation.

**✓ Correct Answer: B. BCC**