A conjunction is a word which is used to connect sentences or clauses in the same clause.Option D) whether is correct as it means expressing a doubt or choice between alternatives and best suits the given sentence. The other options are wrong as if, that, and when indicate (introducing a conditional clause) on the condition or supposition that; in the event that,used to identify a specific person or thing observed or heard by the speaker, and at or during the time that respectively. The correct answer is D) whether.