Explanation:

- Pellagra is primarily caused by a deficiency of Vitamin B3, also known as niacin.

- This condition is characterized by diarrhea, dermatitis, and dementia, and can be fatal if left untreated.

- Option:2 - B3 is the correct answer for the vitamin whose deficiency causes pellagra.

- Vitamin B1, or thiamine, deficiency leads to beriberi, a condition affecting the cardiovascular and nervous systems.

- Vitamin B2, or riboflavin, deficiency causes ariboflavinosis, with symptoms like sore throat and swelling of mucous membranes.

- Vitamin B5, or pantothenic acid, deficiency is rare but can cause symptoms like fatigue and irritability.