Final answer:
The dancer's description sets a mood of wildness and spontaneity, indicating the guests' uninhibited behavior at the party. Therefore, the correct option is A.
Step-by-step explanation:
The description of the dancer in the passage contributes to the mood by demonstrating that the party guests act more wildly than they normally would. The colorful imagery of the gypsy 'in trembling opal' seizing a cocktail and dancing 'like Frisco' on the platform captivates the audience and signifies the commencement of the evening's revelry. This sudden burst into dance suggests a spontaneous and uninhibited atmosphere, where the usual social conventions are relaxed or discarded, enabling a wilder expression among the guests.