Beriberi is a nervous system ailment caused by a deficiency of ------------ in the diet.

Beriberi is a nervous system disorder caused by thiamine (vitamin B1) deficiency. This vitamin is essential for carbohydrate metabolism and proper nerve function. Symptoms include muscle weakness, nerve damage, and cardiovascular problems. The other options (vitamins D, C, E) are not associated with beriberi - their deficiencies cause different conditions like rickets, scurvy, or neurological issues respectively.