Question

complete the sentence with the best relative pronoun.
the term iceberg describes an enormous floating mass of ice 　has broken off a glacier.

Explanation:

We need to choose a relative pronoun to introduce the relative clause that modifies "an enormous floating mass of ice". The relative pronoun should act as the subject of the verb "has broken off", and since the antecedent "an enormous floating mass of ice" is a thing, the appropriate relative pronoun is "that" or "which". In this case, "that" is commonly used in defining relative clauses (which this is, as it's giving essential information about the iceberg).

Answer:

that