Which JTIDS/MIDS access technique is not operator selectable and allows different users in various NPGs to communicate?

The correct answer is multi-nets, as this access technique is inherently designed to facilitate communication among different users operating on various Network Participation Groups (NPGs) without the need for manual selection by the operator. Multi-nets enable simultaneous communications across multiple networks, allowing different users to share information and data efficiently within the Link-16 framework.

This method enhances interoperability and operational flexibility, as it does not require operators to choose specific channels or slots; instead, the system automatically manages the communication across the different NPGs. Operators benefit from multi-nets by being part of a collaborative environment, which is crucial in joint operations where multiple entities need to communicate effectively and in real time.

In contrast, the other access techniques listed involve some level of operator selection or configuration. Dedicated Timeslot Reuse allows operators to manage their access within specific timeslots, Contention access relies on users to compete for access in an uncoordinated manner, and Timeslot Reallocation would involve procedural adjustments that typically necessitate operator choice to optimize the use of available resources.