The best answer is:
B) <span>Earth’s recent warming must be due to something other than changes in the energy output of the Sun.
The paragraph indicates that, although changes in the sun's energy output does affect the temperature on Earth, the changes have been slight and, over the last 30 years in which the Earth has heated up significantly, the sun's energy output has varied only slightly, and has gone in both directions. The implication is that the change in the Earth's temperature must be due to something else. </span>
A, it is informal. She is writing to somebody she personally knows, like her cousin, and doesn't mind if she gets a bad impression from it. Her cousin knows her well enough to not judge her.
The use of the prepositional phrase in this sentence is adjective, because it is part of the noun phrase - the basketball on the roof.
If it weren't a part of the phrase, it would probably be an adverb, but this way, it is not.
