The answers are B: Adverbial clause and C: dependent clause.
A group of words that has the role of an adverb is called Adverbial clause. The sentence "Now if there was one thing that the animals were completely certain of" equals to: Certainly (adverb); or Completely certain (adverb modifying an adjective) and it is both an adverbial clause and a dependent clause (one tha cannot stand by itself).