It is possible for a body to move with a non-zero velocity even when the net zero external unbalanced force is acting upon it.There is no external unbalanced force acting upon it. If the body continues to move with a constant velocity in the same direction, then it will continue to be in a state of motion. 

**Explanation:**

When an object experiences a net zero unbalanced force, it means that the sum of all the forces acting on the object is equal to zero. In this case, there is no overall force acting on the object, which means there is no force to cause any change in its motion. This leads to the conclusion that the object will remain at rest if it was initially at rest or will continue to move with a constant velocity if it was already in motion.

Let's analyze each option to understand why the correct answer is option 'D':

**a) Cannot remain at rest:**
If an object experiences a net zero unbalanced force, it can remain at rest. Without any external force acting on it, the object will continue to stay in its initial state of rest.

**b) Can be accelerated:**
Acceleration is directly proportional to the net force acting on an object. If the net force is zero, the acceleration will also be zero. Therefore, in the absence of a net force, the object cannot experience any acceleration.

**c) Moves with a constant velocity:**
When an object experiences a net zero unbalanced force, it means that the forces acting on it are balanced. In this case, the object will continue to move with a constant velocity if it was already in motion. This is because there is no unbalanced force to change its velocity.

**d) Will always remain at rest:**
This is the correct answer. When an object experiences a net zero unbalanced force, it means that the sum of all the forces acting on the object is zero. Without any unbalanced force, there is no reason for the object to start moving. If the object was initially at rest, it will remain at rest.

Therefore, the correct answer is option 'D' - the object will always remain at rest if it experiences a net zero unbalanced force.

If unbalanced force is net zero then there is no external force acting on the object as opposition so it will move with the constant velocity.