Final answer:
The best control for MilTech defense contracting company to protect against insider attacks is D. Background checks, as they identify potential risks before hiring.
Step-by-step explanation:
The MilTech defense contracting company is seeking to implement an administrative security control that is specifically designed to protect against insider attacks. The most suitable option from the given list would be D. Background checks. Background checks are a proactive measure that helps to verify the credentials and history of employees to prevent hiring individuals who may pose a risk to company security. On the other hand, penetration tests and vulnerability scans are more technical controls focused on identifying security weaknesses from an external perspective, while a data loss prevention system is designed to protect against unauthorized data transfer which could be used as an additional measure, but it does not address the root of the insider risk like background checks do.