The correct option is B. Relative clause refers to a clause that is linked to an antecedent by a relative pronoun such as that, who, which, etc. Looking at the sentences given above, it can be seen that in option B, the two sentences are joined together by the relative pronoun 'that'.
The revision that uses a relative clause to combine the sentences is B: Koalas are marsupials that live in eucalyptus forests. This a restrictive relative clause because it gives essential information with respect to the noun it modifies. If the relative clause was removed, the meaning of the sentence would be altered. Take a look at (1)
1)Koalas are marsupials
The fact is that the sentence states that koalas are marsupials but they are not any type of marsupials but they belong to the kind that lives in eucalyptus forests. Therefore, the information given by the relative clause is essential to identify these marsupials.
