B

Step 1: Understand the basic function of a compound microscope. A compound microscope typically consists of two types of lenses: an objective lens and an eyepiece lens.

Step 2: Identify the characteristics of the objective lens. The objective lens is designed to gather light and focus it to produce a magnified image of the specimen being observed. For effective magnification, the objective lens should have:
- Small focal length: This allows for greater magnification as it can produce a larger image from a smaller object.
- Large aperture: A larger aperture allows more light to enter, improving image brightness and clarity.

Step 3: Analyze the options:
- Option A: A concave lens is not suitable for the objective lens as it cannot converge light to form an image.
- Option B: A convex lens with a small focal length and large aperture is ideal for the objective lens of a microscope.
- Option C: While a convex lens with a large focal length and large aperture can perform well, it is better suited for lower magnification.
- Option D: A convex lens with a small focal length but a small aperture would not allow enough light to form a clear image.

Conclusion: The best option that describes the objective of a compound microscope is Option B: Convex lens of small focal length and large aperture.