He net force acting on an stationary object is zero. which best describes the object’s velocity?
the velocity is zero.
the velocity is decreasing.
the velocity is increasing.
the velocity is constant, but not zero.

When the net force acting on an object is zero, its velocity is constant. 
That means that the object is moving in a straight line with a speed that
doesn't change. 

You said that the object is 'stationary'.  That means that it's not moving. 
So its speed and its velocity are both zero.