The answer is B: Less than a decade ago, the HSUS...an affiliate [Human Society International].
Brackets are used to give the writers (when they need to add information) an opportunity to add an explanation, or emphasis to a word, or change a quote to fit to a sentence, but without changing the meaning of the text. In A) global work doesn't need explanation; C) decade doesn't need explanation; D) HSUS was already explained in the first sentence of the text.