Final answer:
The Shakers failed as a Utopia movement primarily because of their declining membership and difficulty in attracting new members, which was exacerbated by their rule of celibacy that prevented natural population growth.
Step-by-step explanation:
The Shakers, or the United Society of Believers in Christ's Second Appearing, were a utopian religious community known for their celibate lifestyle, equality of the sexes, and communal property. They ultimately failed as a utopia movement primarily due to option A) declining membership and difficulty attracting new members, largely because of their strict rule of celibacy. As fewer individuals joined the Shakers post-mid-century, this critically affected the sustainability and growth of their communities.
Their communities were designed to be self-sustaining havens that separated from secular society, embraced racial equality, and practiced pacifism. Despite having a peak membership of around 6,000 and living communally across different U.S. states, the Shakers could not maintain their population levels because their celibacy rule limited natural population growth, ultimately leading to their decline.