When a particle is revolving in a circular path, it is constantly changing direction which means there is a change in velocity. This change in velocity is called acceleration. The direction of this acceleration is towards the center of the circle and along the radius.

Option 1: Along the tangent - This is incorrect. The acceleration is not along the tangent but along the radius towards the center of the circle.

Option 2: Along the radius - This is the correct answer. The acceleration of a particle revolving in a circular path is directed towards the center of the circle, along the radius. This is because the particle is constantly changing direction and is being pulled towards the center of the circle.

Option 3: Zero - This is incorrect. The particle is experiencing acceleration as it is changing its direction.

Option 4: Along the circumference of the circle - This is incorrect. The acceleration is directed towards the center of the circle, along the radius, and not along the circumference.

In conclusion, the correct option is 2 - along the radius.