The correct answer is C. Capital.
A capital is the enlarged head of a supporting column of a flat slab. It is designed to distribute the load of the slab over a larger area, which helps to prevent the column from cracking. The capital is typically made of the same material as the column, and it is often decorated with moldings or other architectural features.
Option A, supporting end of the column, is incorrect because the supporting end of the column is the part of the column that actually supports the load of the slab. Option B, top of the column, is incorrect because the top of the column is the part of the column that is above the capital. Option D, drop panel, is incorrect because a drop panel is a thickened portion of a slab that is located directly below a column. Drop panels are designed to reduce the bending stresses in the slab at the column support.