Hi Shilpi,

In C, when you say “atmosphere that affects global climate” the meaning conveyed is that it is the atmosphere affecting the global climate. But the intended meaning is that it is the long-term interactions that affect global climate.

In A, when you say “interactions that affect global climate” you are adding some information to the long-term interactions. If you don’t want to repeat the word interactions, then the sentence could be rewritten as “Floating in the waters of the equatorial Pacific, an array of buoys collects and transmits data on long-term interactions between the ocean and the atmosphere, those that affect global climate”. But this sentence has a pronoun ambiguity problem. “Those” could refer to both buoys and interactions