Final answer:
Southern Democrats attempted to block Hiram Revels from his U.S. Senate seat by claiming he hadn't been a U.S. citizen for the requisite nine years, arguing African American citizenship only started with the 14th Amendment in 1868. However, Revels took his seat despite this challenge.
Step-by-step explanation:
When Hiram Revels, the first African American U.S. Senator, was elected in 1870, some Southern Democrats sought to block his Senate seat. However, none of the reasons provided in the options were cited by his opponents; instead, they claimed he had not been a citizen for nine years before his election, as required by the Constitution. The argument was made that since African Americans were not legally recognized as citizens until the passage of the 14th Amendment in 1868, Revels could not have been a citizen for the required duration.
This challenge was not based on Revels's reading ability, educational background, land ownership, or holding of office. Nevertheless, Revels's credentials and his significant political and religious contributions were duly recognized, and he was eventually sworn in, serving as a symbol of progress during the Reconstruction era. His presence in the Senate was a powerful statement against the prevailing racial prejudices of the time.