Final answer:
The best word to replace 'structure' in the given sentence is 'organize', because it accurately conveys the meaning of planning or arranging the fundraising event.
Step-by-step explanation:
The committee wanted to structure the fund-raising event around a good cause. In this context, considering the multiple meanings of the word structure, the right choice to replace it in the sentence would be organize. Option C, organize, best fits as it refers to arranging or putting together the components of the event purposefully around a central theme or cause. Other options such as building, assembly, and maintaining structure as is do not fit the context of planning an event. Therefore, the correct answer is C. organize.