**Core Concept**
Basal-cell carcinoma is a type of skin cancer that originates from the basal cell layer of the epidermis. It is the most common form of skin cancer, particularly in sun-exposed areas, and can occur on the eyelids.

**Why the Correct Answer is Right**
Basal-cell carcinoma is the most common malignant tumor of the lids due to the high exposure to UV radiation in this area. The eyelids have a high concentration of hair follicles, sebaceous glands, and sweat glands, which are all potential sites for basal-cell carcinoma development. The tumor grows slowly and locally, often infiltrating surrounding tissues but rarely metastasizing. This characteristic makes it challenging to diagnose, especially in its early stages.

**Why Each Wrong Option is Incorrect**
**Option B:** Squamous cell carcinoma is a more aggressive form of skin cancer that originates from the squamous cell layer of the epidermis, but it is less common than basal-cell carcinoma on the eyelids.
**Option C:** Adenocarcinoma is a type of cancer that originates from glandular tissue, but it is not the most common malignant tumor of the lids.
**Option D:** Melanocarcinoma is a type of skin cancer that originates from melanocytes, but it is relatively rare compared to basal-cell carcinoma on the eyelids.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a suspected malignant tumor of the lids, it is essential to consider the patient's history of sun exposure, skin type, and previous skin cancers. A thorough physical examination, including a detailed assessment of the eyelid margins, is crucial for early detection and treatment.

**Correct Answer:**
✓ Correct Answer: A. Basal-cell carcinoma