The correct answer is "Whatever was behind them"
Explanation:
A noun clause is a dependent clause (a phrase that depends from another sentence and that adds extra information to it) that works as a noun in a sentence, this means a noun clause is used to name something. Additionally, as dependent clauses, noun clauses are linked to the main sentences by words such as when, what, whether, whatever, why, among others. In the case of "whatever was behind them" this is a dependent clause because it contains a subordinating conjunction "whatever", is part of a complete sentence and depends on it "They froze whatever was behind them stopped moving, too" and it is used to name something in the phrase, in this case, it is used to name an entity or something that stop moving, which means it is not only a dependent clause but a noun clause.
