A company is looking for a hardware feature with the least amount of downtime in terms of firmware upgrades. Which of the following features BEST meet this requirement?

In-service upgrades generally refer to performing maintenance while a system is running, but this term does not inherently guarantee the least downtime without a specific strategy like rolling upgrades, as individual component downtime may still occur.

Rolling upgrades are a strategy where components of a system are upgraded sequentially, one at a time, while the rest of the system continues to operate, thereby minimizing overall downtime or achieving zero downtime for the entire service. This is commonly applied to clusters of servers, storage arrays, or network devices to perform firmware updates without service interruption.

Failover upgrades involve switching to a redundant system during an upgrade, which can achieve minimal downtime, but rolling upgrades often provide a more granular, controlled, and potentially zero-downtime approach for systems composed of multiple active nodes.

User-mode upgrades refer to software upgrades performed within the user space of an operating system, which is not a hardware firmware feature and does not address hardware firmware upgrade downtime.