Final answer:
The most likely diagnosis is Pemphigus vulgaris, an autoimmune disorder characterized by epithelial basal layer separation from the lamina propria, leading to intraepithelial blisters.
Step-by-step explanation:
The most likely diagnosis of an ulcerated gingival lesion with biopsy report confirming epithelial basal layer separation from the lamina propria is C. Pemphigus vulgaris. This condition is an autoimmune disorder characterized by such separation, also known as acantholysis, which leads to the formation of intraepithelial blisters in the mucous membranes and skin. Pemphigus vulgaris is one of the pemphigoids that presents with erosions and ulcerations in the oral cavity, which can be painful and may interfere with eating and speaking.