The dependent clause in the sentence is:
- because it lasted until the third day of June
The other options are not dependent clauses:
- must have been well-packed is an independent clause.
- that James built is a relative clause (but not a full dependent clause on its own).
- The snow fort that James built is an independent clause (it serves as the subject of the main clause).
So, the correct response is because it lasted until the third day of June.