The side that is the altitude is D. bar (EB).
What is an altitude?
An altitude in geometry is a line segment that connects a vertex of a triangle to the perpendicular line opposite that vertex. In the given diagram, the altitude is bar (EB).
Bar (BI) is the base of the triangle. The base is the side opposite the right angle in a right triangle.
Bar (K) is the hypotenuse of the triangle. The hypotenuse is the longest side in a right triangle.
Bar (KE) is a leg of the triangle. The legs are the two sides that meet at the right angle in a right triangle.
Therefore, the altitude is bar (EB).