Final answer:
The Department of Homeland Security is primarily responsible for monitoring and keeping the nation secure from threats, in addition to controlling various law enforcement agencies.
Step-by-step explanation:
The department in the executive branch responsible for keeping our nation secure from threats is the Department of Homeland Security. This department has many duties, including guarding U.S. borders. It also has control over various law enforcement agencies, such as the Coast Guard, the Secret Service, and U.S. Customs.
While the Department of Defense is tasked with military operations and protecting the United States by maintaining armed forces, the Department of Homeland Security is specifically focused on domestic security and protecting the territory of the United States from both natural and human threats.
Other departments such as the Department of State and the Department of Justice also play a role in foreign and domestic policy, but the Department of Homeland Security is the primary department dedicated to the security of the nation's borders and internal safety.