I just took the quiz, answered A on this question, and got a 100%. (On Edge 2021, AP Environmental Science). None of the other answers could cause a greenhouse effect, which is what they want to make sure you understand. The clouds in the troposphere keep some of the outbound UV radiation (after the Earth's surface reflects it) from leaving the Earth, which is why it keeps the Earth warmer.
The green houses gases concentration increases in the the troposphere layer of the atmosphere due to which the sunrays (IR radiation) reflected back by earth are restricted by the green house gases thereby increasing the temperature of the earth and make the climate warmer.
