**Core Concept**
The question is testing for the clinical manifestations of a specific vitamin deficiency, which primarily affects the mucocutaneous surfaces and has systemic effects.

**Why the Correct Answer is Right**
The correct answer is **B. Riboflavin (Vitamin B2)** deficiency. Riboflavin deficiency leads to ariboflavinosis, characterized by lesions of the mucocutaneous surfaces of the mouth and skin, including cracks at the corners of the mouth, glossitis, and dermatitis. Corneal vascularization, anemia, and personality changes are also associated with riboflavin deficiency. This is due to the essential role of riboflavin in energy production and the maintenance of the integrity of mucocutaneous surfaces.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine (Vitamin B1) deficiency is primarily associated with beriberi and Wernicke-Korsakoff syndrome, which does not match the described clinical manifestations.

**Option C:** Niacin (Vitamin B3) deficiency leads to pellagra, characterized by the "three Ds": diarrhea, dermatitis, and dementia, which is distinct from the described clinical picture.

**Option D:** Folate deficiency is associated with megaloblastic anemia, neurological symptoms, and gastrointestinal disturbances, but not the specific mucocutaneous lesions and corneal vascularization described.

**Clinical Pearl / High-Yield Fact**
Ariboflavinosis can be prevented or treated with dietary supplements of riboflavin, and it is essential to recognize this condition to avoid progression to more severe systemic manifestations.

**Correct Answer:** B. Riboflavin (Vitamin B2) deficiency.