Respuesta :
The answer to your question would be that the sentence that uses a correct subordinate conjunction to join the sentences in question is the following one: Since we want to go to the fair, we're going to have to save our money. That is, your answer would be A.
In this case, the subordinating conjunction "since" means because, for the reason that. The reason why they have to save their money is because they want to go the fair.