The answer is (b): rise slowly to a maximum value and drop back rapidly to a slightly smaller value after the block begins to move, as seen in an mpeg video by clicking your mouse on the photograph below.
The coefficient of static friction is greater than the coefficient of kinetic, or sliding, friction. Therefore the greatest force is required to start the motion by overcoming the initially static friction. After the block is moving, the (kinetic) friction becomes less, so the force required to maintain the small but constant velocity becomes less, as can be seen in the video.
