Introduction:
A device used to image the sun is essential for studying its characteristics and phenomena. Among the options provided, the most suitable device for imaging the sun is a pinhole camera. Let's explore the reasons why the other options are not suitable and why a pinhole camera is the ideal choice.

Explanation:

1. Plane Mirror:
A plane mirror reflects light, but it does not form an image of the sun. It simply reflects the sunlight, and the observer can see the reflection of the sun in the mirror. However, this reflection does not provide a clear or detailed image of the sun.

2. Glass Slab:
A glass slab is transparent and does not form an image of the sun. It may refract the light rays from the sun, but it does not create a focused image. Therefore, a glass slab is not suitable for imaging the sun.

3. A Straight Pipe:
A straight pipe, on its own, cannot be used to image the sun. It does not have any optical properties to focus or manipulate light rays to form an image. Therefore, a straight pipe is not suitable for imaging the sun.

4. Pinhole Camera:
A pinhole camera is a simple device that can be used to image the sun. It consists of a light-proof box or container with a small pinhole on one side and a screen or photographic film on the opposite side. When sunlight passes through the pinhole, it forms an inverted image on the screen or film.

The pinhole acts as a small aperture, allowing only a narrow beam of light to pass through. This beam of light creates an image by passing through the pinhole and spreading out on the screen or film. The resulting image is a projection of the sun, with its details and characteristics.

Advantages of a Pinhole Camera:
- A pinhole camera is simple to construct, requiring only a light-proof box, a pinhole, and a screen or film.
- It does not require any lenses or complex optical elements.
- The image formed by a pinhole camera is clear and detailed.
- It can be used to study the sun's features, such as sunspots and solar eclipses.

Conclusion:
Out of the given options, a pinhole camera is the most suitable device to image the sun. It allows the formation of a clear and detailed image of the sun, making it a useful tool for studying its characteristics and phenomena.