The correct answer is (a). The image formed by an astronomical telescope is virtual and diminished.
An astronomical telescope is a type of refracting telescope that is used to view distant objects, such as stars and planets. It consists of two lenses: an objective lens and an eyepiece lens. The objective lens is the larger lens, and it is located at the front of the telescope. The eyepiece lens is the smaller lens, and it is located at the back of the telescope.
When light from a distant object passes through the objective lens, it is refracted (bent) and focused to form a real image. This real image is then located behind the objective lens. The eyepiece lens then magnifies this real image, and the observer sees a virtual image that is located in front of the eyepiece lens.
The image formed by an astronomical telescope is virtual because the light rays do not actually come from the image. The image is also diminished because the size of the image is smaller than the size of the object.
The other options are incorrect. Option (b) is incorrect because the image formed by an astronomical telescope is virtual, not real. Option (c) is incorrect because the image formed by an astronomical telescope is diminished, not magnified. Option (d) is incorrect because the image formed by an astronomical telescope is virtual, not real.