A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with the distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then distance d in cm will be

The correct option is B. 15.
In the absence of a concave lens, the parallel beam will be focussed at the focus $F_2$ of the convex lens i.e. at a distance of 20 cm. from lens A. The focal length of the concave lens is 5 cm. i.e. if this lens is placed at 5 cm from $F_2$ towards the left, then the beam will become parallel.
From figure, it is clear that separation between lenses
d = 20-5 = 15 cm.