Final answer:
The sentence implies that certain social norms and etiquette that were once prevalent no longer hold the same weight in modern times, suggesting a shift in cultural values and social conduct.
Step-by-step explanation:
The sentence 'Rigid rules of courtesy that had once prevailed' suggests that there were social norms and etiquette which were once widely accepted and practiced within a culture or society, but the phrasing implies that these may have since evolved or are no longer as strictly observed. Based on the context given, the correct answer to match with the provided sentence would be B) no longer apply in modern times. Etiquette comprises various forms of social conduct, such as dressing appropriately or dining manners, which contribute positively to quality of life and help maintain a sense of order and cohesion within a community. These norms are guides to behavior that define what is considered good, right, and important within a society. Over time, as societies change and grow more complex, the nature and strictness of these norms can evolve, and actions that were once considered vital for social harmony may become relaxed or interpreted differently, reflecting shifts in cultural values and practices.