Computers mounted on the firefighting apparatus ready for immediate use are called:

The term that accurately describes computers mounted on firefighting apparatus and prepared for immediate use is "Mobile data terminals." These devices are specifically designed to provide firefighters with immediate access to crucial information, such as maps, building layouts, and incident data while they are en route to or at the scene of an emergency. Mobile data terminals facilitate real-time communication and improve situational awareness, making them an essential component in modern firefighting operations.

Onboard computers might suggest a broader range of computer systems within the apparatus that are not primarily focused on data communication during emergency responses. Field communication devices generally refer to radios or other devices used for communication but do not specifically capture the comprehensive data capabilities of mobile data terminals. Emergency response systems typically encompass a larger framework that includes various technologies and methodologies for response management and may not specifically refer to the mounted computers.

In understanding the role and functionality of these terminals, it's clear why Mobile data terminals is the most appropriate answer to the question regarding the computers designed for immediate access and use in firefighting scenarios.