Final answer:
The frequency of sexual activity among heterosexual couples in midlife depends on various factors, including marital status, as married couples have been shown to have sex more frequently than singles.
Step-by-step explanation:
The question regarding surveys reveal that the frequency of sexual activity among heterosexual couples in midlife has multiple choice answers indicating whether this frequency increases, remains constant, decreases, or depends on marital status. Based on the information provided and general trends, it can be said that the frequency of sexual activity does indeed vary among different groups and circumstances. However, as per the depicted trends and the lack of specific information tying frequency directly to midlife, the most fitting answer from the options provided, considering the research on marital status influencing sexual frequency and the lack of clear data specifically tied to midlife, is D. Depends on marital status. This is because recent trends, as noted in the provided information, do highlight that married couples have sex more frequently than singles, which suggests that marital status does play a role in the frequency of sexual activity.