Answer:
The luggage was not misplaced, although it was kept deliberately.
Explanation:
The original sentence is a compound sentence, in which we have two independent clauses joined by the coordinating conjunction "but". To transform it into a complex sentence, we need to change one of the clauses to make it a dependent clause. To do that, we can simply substitute "but" for a subordinating conjunction. A possibility is using "although", which conveys the same idea of contrast:
The luggage was not misplaced, although it was kept deliberately.
