The correct option is C Since
A subordinate clause ("there are five of us") depends on the main clause ("there will not be space for all in the car") for its full meaning. In the given sentence, the subordinate/dependent clause is the cause or reason (more people than the number that can fit in the car) for which the effect is seen in the main clause "there will not be space for all". Hence a conjunction that represents the cause is required. The underlined word 'though' refers to contrast, where the clauses in a sentence have opposite or contrast meaning, and the given sentence isn't a contrast. Hence 'though' needs to be replaced, and option D is incorrect. "Since", in the context of the sentence, refers to the cause, or justification for an action or event "there won't be enough space", hence it's the appropriate conjunction, option C is correct. 'Besides' is used to introduce an additional clause apart from the main clause, and the given statement does not show a mere connection or addition between the clauses, hence option A is incorrect. 'While' is a conjunction used for time-related sentences, that refer to two events taking place at the same time. The given statement does not focus on time, but cause and effect, hence option B is incorrect.