Final answer:
The correct option is (d) 25.
The distance d between the convex lens A and the concave lens B should be 25 cm to allow parallel rays of light entering lens A to exit lens B as a parallel beam. This is because the image from the convex lens must coincide with the focal point of the concave lens.
Step-by-step explanation:
To determine the distance d between a convex lens A with a focal length of 20 cm and a concave lens B with a focal length of 5 cm, where parallel rays of light entering A leave B as a parallel beam, we must consider the lensmaker's equation and the concept of the optical system acting as a combination of lenses.
A parallel beam of light entering the convex lens will converge at its focal point 20 cm away on the other side. At this point, to leave the concave lens as a parallel beam, the light must appear to be emanating from the focal point of the concave lens. So, the image formed by the convex lens must be located at the principal focus of the concave lens. Since the focal length of the concave lens is 5 cm, but with a negative value because it is a diverging lens, this means the image formed by the convex lens A should be 5 cm in front of concave lens B.
Therefore, the distance from the convex lens to the concave lens must accommodate the 20 cm focal distance of lens A and the additional 5 cm to place the image at the focal point of lens B, totaling 25 cm. Hence, the answer to the student's question is (d) 25 cm.