Explanation:

When comparing two concave mirrors with the same focal length but different apertures, the mirror with the larger aperture will form a sharper image.

Reason:
The sharpness of an image formed by a concave mirror depends on the amount of light rays that converge to a single point on the image plane. A larger aperture allows more light rays to enter the mirror, resulting in a greater number of rays converging to form the image.

Aperture:
The aperture of a mirror refers to the size of the opening through which light enters the mirror. It is usually represented by the diameter of the mirror.

Focal length:
The focal length of a mirror is the distance between the mirror and its focal point. It determines the level of convergence or divergence of light rays after reflection.

Image formation:
A concave mirror is a converging mirror, meaning that it brings parallel rays of light to a focal point. When an object is placed in front of a concave mirror, the mirror reflects the light rays and forms an image.

Effect of aperture size on image formation:
- A larger aperture allows more light rays to enter the mirror, resulting in a brighter image.
- With a larger aperture, the light rays from different parts of the object have a wider range of angles, allowing for a more accurate convergence of rays at the focal point.
- This leads to a sharper and more detailed image formation.

Comparison of options:
a) Plane mirror: A plane mirror does not have a focal length or an aperture. It forms a virtual, upright, and laterally inverted image of the object. It is not relevant to the comparison of aperture size and image sharpness.
b) Concave mirror: As explained above, a concave mirror with a larger aperture will form a sharper image due to the increased number of converging light rays.
c) Convex mirror: A convex mirror is a diverging mirror that forms virtual, diminished, and upright images. It is not relevant to the comparison of aperture size and image sharpness.
d) Prism: A prism is a transparent object that can refract and disperse light. It does not have a focal length or an aperture, and it does not form images. It is not relevant to the comparison of aperture size and image sharpness.

Hence, the correct answer is option B - Concave mirror.