**Explanation:**

To understand why the torque on the stone is zero, let's break down the concept step by step.

**1. Torque:**
Torque is defined as the rotational equivalent of force. It describes the tendency of a force to rotate an object about an axis. Mathematically, torque is given by the equation:

Torque (τ) = r * F * sin(θ)

where r is the perpendicular distance from the axis of rotation to the line of action of the force, F is the magnitude of the force, and θ is the angle between the force vector and the line connecting the axis of rotation and the point of application of the force.

**2. Circular Motion:**
In circular motion, an object moves along a circular path with a constant speed. The object experiences a centripetal force directed towards the center of the circle, which provides the necessary inward acceleration to keep the object moving in a circle.

In this scenario, the stone tied to the string is rotating along a circular path. The tension in the string provides the centripetal force required for the circular motion.

**3. Torque on the Stone:**
In the given scenario, the stone is rotating with a constant speed v along a circular path of radius l. The tension in the string exerts a force towards the center of the circle, i.e., along the radial direction.

Since the force is acting along the radial direction, the angle θ between the force vector and the line connecting the axis of rotation and the point of application of the force is zero degrees. Therefore, sin(θ) = 0.

Substituting sin(θ) = 0 into the torque equation, we get:

Torque (τ) = r * F * sin(θ)
= r * F * 0
= 0

Thus, the torque on the stone is zero.

**Conclusion:**
The correct answer is option 'A' (zero). Since the force responsible for the circular motion is acting along the radial direction, the torque on the stone is zero.