The correct option is At the principal focus.
Explanation
Image formation in spherical mirrors is governed by the laws of reflection. A concave mirror is a converging mirror, meaning it brings parallel light rays together at a specific point. The position and nature of the image formed depend on the distance of the object from the mirror.
Analysis of Image Formation:
- When an object is located at infinity, the light rays arriving at the mirror are considered parallel to the principal axis.
- Upon reflection, all rays parallel to the principal axis converge at a single point on the principal axis known as the Principal Focus (F).
- Consequently, the image is formed at the focus.
- The characteristics of the image formed in this specific configuration are:
- Position: At the principal focus (F).
- Nature: Real and Inverted.
- Size: Highly diminished (point-sized).
Comparison with Other Positions:
- At the centre of curvature: The image forms here only if the object is placed at the centre of curvature.
- At twice the focal length: This is synonymous with the centre of curvature ($R = 2f$). The image forms here only if the object is at $2f$.
Key Takeaway:
For a concave mirror, incident rays from an object at infinity are parallel to the principal axis and converge to form a real, point-sized image at the principal focus.