Question

He has been working very hard to clear the entrance examination. (rewrite as a
complex sentence using an adverbial clause)

Answer 1

a complex sentence is a sentence containing a subordinate clause or clauses.
the answer-
Because he must clear the entrance exam, he has been working very hard. 

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Answer 2

Complex Sentence - Adverbial Clause

He has been working hard so that he could clear the entrance examination.