Which network supports classified email, internet, and tactical chat, and is a subnetwork of DISN?

The correct choice pertains to SIPRNET, which stands for Secure Internet Protocol Router Network. This network is specifically designed to handle classified information, providing secure access for email, internet use, and tactical chat capabilities for users authorized to access sensitive but unclassified information. As a subnetwork of the Defense Information Systems Network (DISN), SIPRNET plays a crucial role in ensuring that military and government personnel can communicate securely without compromising sensitive information.

SIPRNET's design allows for secure exchanges that comply with the rigorous standards of handling classified data, thus ensuring the integrity and confidentiality of communications within the military and related operations. It is essential for maintaining operational security while facilitating necessary communications that are otherwise not safe to transmit over unclassified networks.

Other networks listed serve different purposes or levels of security and do not encompass the same breadth of support for classified email and tactical chat capabilities as SIPRNET. This distinction highlights why SIPRNET is the appropriate choice in this context.