Final answer:
The Shakers are the religious sect known for their plain and functional interior and furniture design, emphasizing simplicity and craftsmanship.
Step-by-step explanation:
The religious sect known for developing very plain, functional interiors and furniture is the Shakers. Contrary to the extravagance of Victorian style, the Shakers emphasized simplicity, functionality, and craftsmanship in their design. This ethos stemmed from their desire to live humble, religious lives devoid of excess.
The Shakers built their communities and belongings with a focus on communal living and the utility of every object. Their beliefs in simplicity and function influenced American design, leaving a legacy that is still appreciated today. Shaker furniture, for instance, is recognized for its clean lines and quality construction, elements that have become staples in the design world.
Other religious groups like the Puritans, Amish, and Mennonites also pursued similar ideals of architectural simplicity, but the Shaker community's impact on design aesthetics stands out historically and socially.