You are implementing shaders into a 3D game-rendering engine.
What describes a shader?

A.
it is a programmable interface before the graphics pipeline.

B.
It provides a programmable interface into the graphics pipeline.

C.
it is a transformation scripting interface.

D.
it is a scripting graphical user interface.

Explanation:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb944006(v=vs.85).aspx