Which aspect is critical for maintaining service availability during routine maintenance?

Maintaining service availability during routine maintenance is primarily supported by high availability setups. High availability refers to systems designed to ensure maximum operational uptime, effectively minimizing downtime during maintenance activities. By incorporating redundancy through clusters, failover mechanisms, and load balancing, high availability setups can continue delivering services even when parts of the system are being maintained or are temporarily offline. This approach is crucial in cloud environments where continuous access to applications and services is expected by users.

In contrast, the other options address important aspects of infrastructure but are not as directly related to maintaining service availability during maintenance. For instance, increased server resources can enhance performance but does not inherently ensure availability. Load distribution helps balance workloads but might not be sufficient if a primary server goes down. Similarly, backup power supplies are essential for preventing service interruptions due to power issues, yet they do not specifically tackle the challenges associated with performing maintenance on existing services. Thus, high availability setups are the best solution for ensuring that services remain accessible even while maintenance is occurring.