Choose the right word to complete the student's highly suspicious excuse.

_____ the aliens promised to return my math homework, I suspect they used it as spaceship fuel.

The correct answer is Although.

Although is a subordinating conjunction used to introduce a dependent clause that contains a subject and a verb ("the aliens promised"). However and nevertheless are conjunctive adverbs and cannot connect a dependent clause to an independent clause in this way. Despite is a preposition and must be followed by a noun or a gerund, not a full clause.