Not necessary, because the negation of (A) arguably helps the argument. If NO influential members of the committee are interested in the savings available by renting the equipment today, that suggests further reason to think the committee doesn’t care much about saving money.

Not necessary, because we already have a premise establishing that the club’s party committee “knows the costs involved.” This doesn’t require that over half of the members of the committee be aware of the “financial situation” of the club. In addition, “financial situation” is broader than the specific cost savings available concerning the rental equipment. The argument doesn’t require the author to believe anything about the committee’s awareness of the general “financial situation” of the club.

Not necessary, because the argument concerns the potential cost savings from renting at the “usual rental agency.” Other rental agencies are irrelevant.

Not necessary, because (D) arguably weakens the argument if assumed. If the committee has a number of responsibilities that take priority over saving money, that opens the possibility that they DO care about saving money, but they might need to refrain from paying for the equipment today for other, more important reasons.

Necessary, because if it were not true — if there is NOT enough cash available to the glee club today to pay for the equipment rental at today’s lower price — then the fact the committee is not renting the equipment today would not establish anything about the committee’s motivations. It could just reflect inability to pay, not a lack of desire to save.