Objective:
To explain why an astronomical refracting telescope will have large angular magnification and high angular resolution when it has an objective lens of large focal length and large diameter.

Explanation:
An astronomical refracting telescope consists of two lenses: the objective lens and the eyepiece. The objective lens is responsible for gathering and focusing the incoming light, while the eyepiece magnifies the image formed by the objective lens for the observer to see.

Angular Magnification:
Angular magnification is the ratio of the angle subtended by the image seen through the telescope to the angle subtended by the object when viewed with the naked eye. It determines how much larger the object appears when viewed through the telescope.

Angular Resolution:
Angular resolution refers to the ability of a telescope to distinguish between two closely spaced objects. It is a measure of the smallest angle between two objects that can be resolved by the telescope.

Objective Lens:
The objective lens is the primary lens of the telescope and plays a crucial role in determining the angular magnification and angular resolution.

Focal Length:
The focal length of the objective lens determines the angular magnification of the telescope. A shorter focal length will result in a larger angular magnification since the image formed will be larger. On the other hand, a longer focal length will result in a smaller angular magnification.

Diameter:
The diameter of the objective lens affects the angular resolution of the telescope. A larger diameter allows more light to enter the telescope, resulting in a higher resolution. This is because a larger diameter provides a larger aperture, which reduces the effects of diffraction and improves the ability to resolve fine details.

Optimal Combination:
To achieve both large angular magnification and high angular resolution, a refracting telescope should have an objective lens with a large focal length and a large diameter.

- A large focal length will provide a larger angular magnification, allowing objects to appear larger when viewed through the telescope.
- A large diameter will provide a higher angular resolution, allowing the telescope to distinguish between closely spaced objects and resolve fine details.

Therefore, option 'C' - large focal length and large diameter - is the correct answer.