Which document outlines the training objectives for OPFOR?

The Combined Arms Training Strategy (CATS) is specifically designed to provide guidance on training across the various components of military operations, including the objectives for opposing forces (OPFOR). This document emphasizes the integration of combined arms in training exercises, ensuring that OPFOR units are effectively prepared to simulate enemy capabilities and tactics, which is crucial for realistic and effective training for friendly forces.

CATS outlines how to develop training scenarios that reflect current operational environments, enhancing readiness and combat effectiveness. Therefore, it serves as a foundational document in establishing the training objectives that OPFOR should meet, making it the correct answer to the question.

The other options, while related to military operations and training, serve different purposes and do not specifically address the training objectives for OPFOR. The National Defense Strategy and the Military Training Guidance provide broader strategic frameworks and directives, while the Joint Operations Plan focuses on coordination and execution of specific operations, rather than training objectives.