Suppose a body that is acted on by exactly two force is accelerated. Does it then follow that (a) the body cannot move with constant speed; (b) the velocity can never be zero; (c) the sum of the two forces cannot be zero; (d) the two forces must act in the same line?

The body is accelerated by two forces.
(a) The body can move with constant speed.
(b) The velocity can be zero.
(c) The sum of two forces can be zero.
(d) The two forces can act in the opposite direction to each other.
The body can move with constant speed. The resultant of the two forces acting on the body will be the net force. As an example, the push of our hand on the sliding block can exert a force that counteracts the force of friction on the block, and an upward force of the horizontal plane counteracts the force of gravity. Thus the net force on the block can then be zero. It signifies that, the block can move with constant speed and it is possible the body will attain zero velocity. From the above observation we also conclude that, the two forces can act in the opposite direction to each other.