Objective of a Telescope:
The objective of a telescope is to collect and focus light from distant objects so that they can be magnified and viewed by the observer. It is responsible for forming a real and inverted image of the object.

Focal Length of Lenses:
The focal length of a lens is the distance between the lens and the point where the light rays converge or diverge. It determines the amount of bending of light that occurs when it passes through the lens.

Choice of Objective Lens:
To determine which lens to use for the objective of the telescope, we need to consider the characteristics of the lenses.

- The objective lens should have a longer focal length compared to the eyepiece lens. This is because a longer focal length objective lens will produce a larger magnification and a higher quality image.
- The objective lens should also have a larger diameter to collect more light and provide a brighter image.

Comparison of Focal Lengths:
The given focal lengths are 5 cm and 50 cm.

- The 5 cm focal length lens is relatively short and will provide a higher magnification but with a smaller field of view. It will not gather as much light, resulting in a dimmer image.
- The 50 cm focal length lens is longer and will provide a lower magnification but with a larger field of view. It will gather more light, resulting in a brighter image.

Selection of Objective Lens:
Considering the requirements of the objective lens, the 50 cm focal length lens is more suitable for the objective of the telescope. It will provide a larger field of view and a brighter image, which are desirable characteristics for a telescope's objective.

Therefore, the correct answer is option D, i.e., the 50 cm lens should be used for the objective of the telescope.