Answer:
After the ceremony had concluded
Explanation:
In the sentence, there are two clauses: a subordinate or dependent clause (<em>After the ceremony had concluded</em>) and an insubordinate or independent clause (<em>the prom queen cried tears of joy</em>),
The first one is a subordinate clause because it does not express a complete thought by its own, and therefore, it needs to be with another clause in order to make sense. The second one is an insubordinate clause because it has a full meaning on its own: if you were to remove the first clause from the sentence, the insubordinate clause would still make sense.
Furthermore, <em>After the ceremony had concluded</em> is not only a subordinate clause but it is also an adverb clause because it modifies the action of the second clause by adding information of <em>WHEN</em> the prom queen cried tears of joy.