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What is Oral lesions on the tongue and other mucosal surfaces of the oral cavity is caused by the deficiency of: and explain in detail?
A deficiency of niacin leads to the classic symptoms of pellagra in human beings.
Clinical Features of Pellagra:

The mucous membrane lesions affecting the tongue, oral cavity and vagina are usually the earliest lesions diagnostic of the disease.
Other lesions common in pellagra are the typical dermal lesions of bilaterally symmetric, sharply outlined, roughened, keratotic areas. (The word ‘pellagra’ means rough skin). 
Mental symptoms and weight loss also occur.

Ref: Shafer's textbook of oral pathology 7th edition page 646