Final answer:
The lack of formal training and education of law enforcement officers led to the establishment of the Reform Era of policing in the early 20th century, which aimed to professionalize the force with better standards and training. The correct answer is option: Reform Era.
Step-by-step explanation:
The era of policing created because of the lack of formal training and education of law enforcement officers is the Reform Era.
The Reform (or Professional) Era began in the early 20th century as a response to the issues within the policing system that existed during the Political Era, which was characterized by police forces with strong ties to political leaders and little emphasis on professional training or standards.
It was during the Reform Era that a more professional approach to policing was advocated, emphasizing centralized police organizations, higher standards for law enforcement officials, and the introduction of formal training programs.