Answer:
'than the Franks' is the elliptical adverb clause.
Step-by-step explanation:
A adverb clause is a dependent clause that serves as an adverb in a statement. An adverb, in a statement, is used as a modifier of a verb, an adjective, and another adverb. Thus, an adverb clause serves as an modifier of a verb, an adjective, and another adverb in a sentence.
An elliptical adverb clause is the one in which the words are implied. In such clauses, the words are left intentionally to be implied.
In the given sentence, the elliptical adverb clause is 'than the Franks' in which the words that are left to implied mean that the Goths crossed into the Roman territory than the Franks crossed the territory.
Thus the correct answer is 'than the Franks.'