A

Step 1: To determine the point in a vertical circle where the tension is zero for a particle just able to complete the circle, we must consider the forces acting on the particle.
Step 2: The forces acting on the particle at any point in the vertical circle are gravity (weight of the particle, given by mg, where m is mass and g is the acceleration due to gravity) and tension (T).
Step 3: For the particle to just complete the circle, the gravitational force must provide the necessary centripetal force at the highest point of the circular path. The centripetal force required at the highest point is given by the equation:
$$F_c = \frac{mv^2}{r}$$
where v is the speed of the particle at that point.
Step 4: At the highest point, the forces acting on the particle can be represented as:
$$mg - T = \frac{mv^2}{r} $$
If the particle just completes the vertical circle, the tension is zero (T = 0). Thus the equation becomes:
$$mg = \frac{mv^2}{r}$$
Step 5: This shows that at the highest point, the tension can be zero if the gravitational force is sufficient to maintain the centripetal acceleration, confirming Option A: Highest point as the correct answer.