Correct answer: A. It can absorb longwave radiation
Explanation: The direct answer is A, water vapour can absorb longwave radiation. Earth receives mainly shortwave solar energy and sends energy back toward space as longwave terrestrial radiation. Water vapour absorbs much of this outgoing longwave radiation and re-emits energy in different directions, including back toward Earth. This makes it an important natural greenhouse gas and helps regulate the heat budget. Option A is correct. Option B, water vapour is the source of solar energy, is wrong because solar energy comes from the Sun, not from atmospheric moisture. Option C, it forms Earth’s orbit, is wrong because orbit is determined by gravity and motion, not water vapour. Option D, it always makes albedo zero, is wrong because albedo concerns reflection from a surface and is not always zero because of water vapour. The chain is: Earth warms → emits longwave radiation → water vapour absorbs part of it → heat is retained and re-emitted. Memory cue: water vapour traps outgoing heat, not incoming sunlight as its main role.