Final answer:
The primary purpose of Security Classification Guides (SCG) is to provide derivative classification instructions, including classification level, declassification, and downgrading instructions to protect sensitive information.
Step-by-step explanation:
The primary purpose of Security Classification Guides (SCG) issued by an Original Classification Authority (OCA) is to provide derivative classification instructions, which include the classification level, declassification instructions, and downgrading instructions. Option a is the correct answer. Security Classification Guides help in determining how information should be handled and protected based on its level of sensitivity and potential impact on national security. These guides ensure that individuals with access to classified information understand their responsibilities in protecting it, thus preventing unauthorized disclosure.