There are only two adverbs next to each other in the sentence, and those are the words rather and closely. So, the correct answer is rather - that is the adverb that describes another adverb - closely. You can ask the question - How closely?, and you will get the answer - Rather (closely), which describes the adverb closely.
There is another adverb in the sentence - the word increasingly, but it is describing an adjective rare, and not an adverb, which is why it is incorrect.
