Final answer:
The most likely source of threats to data security are authorized users, who may misuse their legitimate access privileges.
Step-by-step explanation:
Threats to data security are most likely to come from authorized users. While external hackers pose a significant risk, it's often the insiders with legitimate access who can cause the most damage, either maliciously or accidentally.
These individuals have legitimate access to the system and can misuse their privileges without the same barriers an external party would face. Meanwhile, security protocols and firewalls are designed to protect data, not threaten it, making options C and D incorrect.