Q: When a man walks on a horizontal surface with constant velocity, work done by
(A) Friction is zero
(B) Contact force is zero
(C) Gravity is zero
(D) Man is zero
Solution : 
Since mg & N are perpendicular to velocity $\displaystyle \vec{v} $
and $ \displaystyle \vec{ds} $, work done by these forces is zero.
Since no relative sliding occurs during walking, static friction comes into play. Hence the point of application of static frictional force does not move.
Therefore no work is done by frictional force.
The man has to lose his body’s (internal) energy E , hence performs work because W = ΔE