The correct answer is option 'A': Plane mirror.

A plane mirror is used in a car's rearview mirror. Let's understand why a plane mirror is chosen for this purpose.

1. Definition of a plane mirror:
- A plane mirror is a flat and smooth reflective surface that reflects light rays uniformly.
- It forms a virtual image that is upright, the same size as the object, and located behind the mirror.

2. Purpose of a rearview mirror in a car:
- The rearview mirror in a car is positioned inside the vehicle, usually on the windshield or the interior roof of the car.
- It allows the driver to see the reflection of the area behind the car without turning their head or diverting their attention from the road ahead.
- It helps the driver to monitor the traffic behind the car and make informed decisions while driving, such as changing lanes or reversing.

3. Advantages of using a plane mirror in a rearview mirror:
- Wide field of view: A plane mirror provides a wide field of view, allowing the driver to see a larger area behind the car.
- No distortion: Plane mirrors reflect light rays without distorting them, providing an accurate representation of the objects behind the car.
- Upright image: The image formed by a plane mirror is upright, which means it appears the same way as the actual object. This helps the driver perceive the distance and position of the objects accurately.
- Same size image: The image formed in a plane mirror is the same size as the object being reflected. This allows the driver to gauge the relative sizes of objects behind the car.

4. Comparison with other mirror types:
- Convex mirror: Convex mirrors are commonly used as side mirrors in cars. They provide a wider field of view but result in a smaller, distorted image. Convex mirrors are preferred for side mirrors as they help eliminate blind spots.
- Concave mirror: Concave mirrors are used in headlights and fog lights of cars to focus and direct the light. They are not suitable for rearview mirrors as they produce a magnified, inverted image.
- Spherical mirror: The term "spherical mirror" is a general term that includes both convex and concave mirrors. Since a plane mirror is a specific type of spherical mirror, it is the correct answer for the given question.

In conclusion, a plane mirror is used in a car's rearview mirror because it provides a wide, undistorted field of view, reflects an upright and same-sized image, and allows the driver to accurately perceive objects behind the car.

Convex mirror is use as rear view mirror in vehicles because it gives wider field of view and forms diminished image of objects.