**Core Concept**
Basal cell carcinoma (BCC) of the eyelid can invade the surrounding orbital tissues, leading to various symptoms. The early detection of orbital involvement is crucial for timely intervention.

**Why the Correct Answer is Right**
Orbital involvement by BCC can manifest as a proptosis due to the tumor's expansion into the orbital fat. This occurs because the tumor grows slowly and infiltratively, causing the eyelid to bulge outwards. The exact mechanism is due to the tumor's invasion into the orbital fat and surrounding tissues, leading to a displacement of the globe.

**Why Each Wrong Option is Incorrect**
**Option A:** Diplopia - While this can be a symptom of orbital involvement, it is not the earliest symptom. Diplopia occurs when the tumor affects the extraocular muscles or the nerves controlling them.
**Option B:** Ptosis - This can be a symptom of orbital involvement, but it is not the earliest symptom. Ptosis occurs when the tumor affects the levator palpebrae superioris muscle or the nerves controlling it.
**Option C:** Enophthalmos - This is the opposite of proptosis and would not be an early symptom of orbital involvement by BCC.

**Clinical Pearl / High-Yield Fact**
Remember that basal cell carcinoma of the eyelid can be mistaken for a benign lesion, such as a chalazion or a benign sebaceous gland tumor. A high index of suspicion and regular follow-up are essential for early detection and treatment.

**Correct Answer:** B. Proptosis.