The correct answer is option 'C': Only if both the curved surfaces are symmetrical.

Explanation:
The optical centre of a lens is defined as the point on the principal axis of the lens where any incident ray passing through it emerges without any deviation. In other words, the optical centre is the point through which a ray of light passes undeviated.

A lens can have different shapes and designs, but for the optical centre to be at the centre of the lens, it is necessary for both the curved surfaces of the lens to be symmetrical.

Here is a detailed explanation:

1. Definition of the optical centre:
- The optical centre is the point on the principal axis of a lens where any incident ray passing through it emerges without any deviation.
- It is the point of intersection of the principal axis with the lens.

2. Symmetrical lens:
- A symmetrical lens is a lens that has the same shape on both sides.
- In a symmetrical lens, the curvature of the surfaces is the same on both sides, resulting in equal focal lengths on either side.

3. Importance of symmetrical curved surfaces:
- When the curved surfaces of a lens are symmetrical, the incident rays passing through the optical centre encounter equal curvature on both sides.
- This symmetry ensures that the rays passing through the optical centre emerge undeviated, as the curvature on both sides cancels out each other's effects.

4. Non-symmetrical lens:
- In a non-symmetrical lens, the curvature of the surfaces is different on both sides.
- Due to the asymmetry, the incident rays passing through the optical centre encounter different curvatures on each side, resulting in deviation.

5. Optical centre not at the centre of the lens:
- In a non-symmetrical lens, the optical centre is not located at the centre of the lens.
- The optical centre is determined by the point on the principal axis where the incident rays pass through undeviated, and this point may not necessarily coincide with the geometrical centre of the lens.

In conclusion, the optical centre of a lens is only at the centre of the lens if both the curved surfaces are symmetrical. If the lens is asymmetrical, the optical centre will not be at the centre of the lens.