The correct answer is A) corruption.
The major weakness of these forces was corruption.
The problem and big issue here were that policemen, police officers were appointed by politicians that most of the time served political interests instead of serving the citizens. This was part of the United States political history from 1840 to 1920, commonly known as the Political Era of Policing. Local politicians hired the police members and of course, the forts served the politician interests which allowed corrupt practices and injustices in society.