Final answer:
The correct answer is B. which, as it introduces a non-restrictive relative clause to provide additional information about hyenas in the given sentence.
Step-by-step explanation:
The correct grammatical choice to fill in the blank in the sentence 'Hyenas _______ are rather small animals, are able to get food through cunning.' is B. which. The sentence should read: 'Hyenas, which are rather small animals, are able to get food through cunning.' The use of 'which' in this context introduces a non-restrictive relative clause, providing additional information about the hyenas. In contrast, using 'that' would imply a restrictive clause, which is not the intention in this sentence because the clause is not essential to the meaning of the sentence.