Final answer:
The engineer is creating a methodology to verify each security control in each unit of developed code. Therefore the correct answer is Option D.
Step-by-step explanation:
The contents of the supporting document the security engineer is creating can be best described as a methodology to verify each security control in each unit of developed code prior to committing the code. This document would outline the process and steps to be followed by the developers to ensure that all security requirements are met. By verifying each security control in the code, the engineer ensures that the system is secure and protected from potential threats.