Which category best describes the positions of civilians working on Top Secret projects in the DOD?

The correct choice is that civilians working on Top Secret projects in the Department of Defense (DOD) are categorized as Critical-sensitive. This classification is used specifically to denote positions that require a high level of security clearance due to the sensitive nature of the work involved, especially in relation to national security and defense operations. Critical-sensitive positions often deal with issues that could significantly impact the security of the United States if compromised, making rigorous screening and oversight necessary for individuals in these roles.

Positions classified as Secret-sensitive or Nuclear-sensitive, while they pertain to important security roles, do not encompass the same breadth of critical responsibilities that come with handling Top Secret information. The term "Top-sensitive" is not a recognized category within the security clearance classifications, which further supports that Critical-sensitive is the term that best encapsulates the nature of these positions dealing with Top Secret projects.