The correct answer is: B. mirror
Large astronomical telescopes always use a mirror as the objective because it is more efficient than a lens. A mirror can reflect all of the light that hits it, while a lens can only transmit a fraction of the light that hits it. This is because a mirror is a solid object, while a lens is made up of two pieces of glass that are separated by air. The air between the two pieces of glass causes some of the light to be lost.
In addition, a mirror can be made much larger than a lens of the same weight. This is because a mirror is supported by its back, while a lens is supported by its edges. The edges of a lens are more likely to break than the back of a mirror.
Finally, a mirror is less expensive to make than a lens of the same size. This is because a mirror can be made out of a single piece of glass, while a lens must be made out of two pieces of glass that are ground and polished to a precise shape.
Option A is incorrect because a lens is not as efficient as a mirror. Option C is incorrect because a combination of lenses is not as efficient as a single mirror. Option D is incorrect because large astronomical telescopes always use a mirror as the objective.