What is the easiest learning curve to develop a Mark Reader software?
EMGU is a .NET wrapper for OpenCV which is a powerful library for computer vision applications.

You could use something like blob detection to set up the layout of your grid of answers in space, check the pencil shading against a threshold (to prevent erasures from being detected as answers), check that there's one answer per question, and that it's in the right spot.

Get some "toy" problems going with the library first (detect a circle or a square in a field) and build up your implementation from there.