To solve this problem, we need to understand the behavior of light passing through a lens system.$\\$The condition given is that a parallel beam of light falling on lens 'A' leaves lens 'B' as a parallel beam.$\\$This implies that the system of lenses is acting as a telescope in normal adjustment, where the image formed by the first lens (convex lens 'A') is at the focal point of the second lens (concave lens 'B').$\\$Let's denote:$\\$• $f_A = 20$ cm as the focal length of the convex lens 'A'.$\\$• $f_B = -5$ cm as the focal length of the concave lens 'B'.$\\$• $d$ as the distance between the two lenses.$\\$For the light to emerge as a parallel beam after passing through both lenses, the image formed by lens 'A' should be at the focal point of lens 'B'.$\\$This means the distance between the lenses $d$ should be equal to the sum of the focal lengths of the two lenses.$\\$Therefore, we have:$\\ d = f_A + |f_B| \\$Substitute the given values:$\\ d = 20 + 5 \\ d = 25$ cm$\\$However, the correct option is given as 15 cm. Let's re-evaluate the condition:$\\$The condition for the lenses to act as a telescope in normal adjustment is that the distance between the lenses $d$ should be equal to the difference of the focal lengths of the two lenses.$\\$Therefore, we should have:$\\ d = f_A - |f_B| \\$Substitute the given values:$\\ d = 20 - 5 \\ d = 15$ cm$\\$Thus, the correct distance $d$ is $15$ cm, which corresponds to Option 2.