Why he didn't jump at the chance of a trip abroad, I really can't imagine.

Explanation

The sentence contains an independent clause:- I really can't imagineAnd a subordinate clause (introduced by the pronoun "why"):- why he didn't jump at the chance of a trip abroadThe subordinate clause cannot stand alone as a complete sentence, and it functions as a question ("Why...") that is being reported or explained by the main clause ("I really can't imagine"). This makes the sentence complex.