A legacy application used for a critical business function relies on software that has reached...

 Segmenting the application within the existing network is the most effective control to manage a legacy application that relies on software that has reached the end of extended support, as it isolates the application from the rest of the network and reduces the attack surface and the potential impact of a compromise. Subscribing to threat intelligence, applying patches for a newer version of the application, and increasing the frequency of regular system and data backups are not the most effective controls, as theymay not address the root cause of the risk, or may introduce additional costs or complexities, respectively. References = CRISC Review Manual, 7th Edition, page 153.