The best response regarding the influence of the historical setting on the characterization of Rosemary Fell in Katherine Mansfield's "A Cup of Tea" is:
The historical setting shows she is a leader in her community.
This is evident in the way she hosts parties that mix important people and artists, suggesting her social status and influence. Her being "extremely modern" and "well-read" also implies that she is engaged with contemporary ideas and trends, positioning her as a figure of leadership and cultural relevance.