Final answer:
The most important assessment when considering the calendar method of contraception is an accurate menstrual cycle diary for the past 6 to 12 months to predict the fertile window and help determine ovulation times.
Step-by-step explanation:
The most important assessment for a nurse to obtain when a client requests information about using the calendar method of contraception is B) An accurate menstrual cycle diary for the past 6 to 12 months. This is essential because the calendar method relies on predicting the fertile window based on past menstrual cycles. It requires the female to have regular, predictable menstrual cycles. An accurate record will help determine the length of her cycles and identify the likely time of ovulation, which is vital for this natural family planning method to be effective.