Option (a): When we are on earth, we can see a rainbow. The process involves refraction and internal reflection of sunlight by water droplets in air. The refracted rays then reach our eyes. Even if we increase our altitude, there still could be refracted rays that can reach our eyes. A plane’s altitude is well within the atmosphere. Hence, this phenomenon is still visible by a passenger in an airplane. Thus, (a) is incorrect.

Option (d):

When some of the light passes through a second water droplet, we get a secondary rainbow. The process has an additional step now than in (a). Using the same reasoning as in the previous paragraph, we can conclude that the passengers can see a secondary rainbow. Thus, (d) is incorrect.

Option(c) and Option (b):

At an altitude, there is no ground ahead of you. Hence, when a rainbow forms, you can see a full circle. On the ground, you may or may not see a full circle depending on the position of the rainbow. However, on a plane, you never see incomplete arcs rather full circle. Hence, option (b) is correct and (c) is incorrect.