A convex lens $A$ of focal length $20\,\mathrm{cm}$ and a concave lens $B$ of focal length $5\,\mathrm{cm}$ 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 $\mathrm{cm}$ will be

A parallel beam incident on the convex lens $A$ would, by itself, form an image at its focal point, so the beam would converge to a point $20\,\mathrm{cm}$ to the right of $A$. For the concave lens $B$ to make the emerging beam parallel, the converging rays reaching $B$ must be directed toward its focal point on the right side. For a concave lens, that focal point is at a distance $5\,\mathrm{cm}$ from the lens. So the point where lens $A$ alone would focus the rays must lie $5\,\mathrm{cm}$ to the right of lens $B$. Hence the separation must satisfy $$d+5=20$$ Therefore, $$d=15\,\mathrm{cm}$$ Now checking the options, $15$ matches option $B$.