In astronomical telescope, the final image is formed at
A. the least distance of distinct vision
B. the focus of objective lens
C. the focus of the eye lens
D. infinity

Hint:The answer can be found by knowing the basic function for where the astronomical telescopes are made and the relation between the distance of image and distance of object and its apparatus that is ray diagram.

Complete step by step answer:
 Telescope is an optical instrument made of lenses of mirrors that is used to see away objects. So, there are two types of telescopes i) Refracting type telescope ii) Reflecting type telescope.Astronomical telescope is a refracting type of telescope made of a combination of two lenses, one objective of large focal length and other eye piece is small focal length as shown in figure. Astronomical telescope is used to see heavenly bodies in the sky like plants, stars etc.

From the diagram, it is clear as in an astronomical telescope we see astronomical objects which are at infinite distance.So, their image is formed at focus of the objective, ${F_0},$ real and inverted. The image acts as an object for eye piece and this telescope is adjusted so that the final image thus formed is at infinity.The final image formed is at infinity and is magnified, virtual and inverted with respect to object.
Hence, the correct option is D.

Note: The final image is virtual and inverted not real and invested as from the diagram, it is clear that the eyepiece forms a virtual and erect image of object AB which is inverted image of real object. So, it is virtual and inverted with respect to the astronomical objects but this is not visible on detestable through astronomical telescopes, because generally the planets are spherical in shape.