The correct answer is A. Generator.

A generator can provide long-term backup power during an extended power outage, as long as sufficient fuel is available. For a multiday outage, a generator is the appropriate solution to keep a data center operational.

This aligns with CompTIA Security+ SY0-701 availability and resilience concepts. Data centers often use generators, redundant power feeds, UPS systems, and environmental controls to maintain uptime.

Why the other options are incorrect:

B. Uninterruptible power supply

A UPS provides short-term backup power and protects against brief outages, power fluctuations, and brownouts. It is usually designed to keep systems running until generator power starts or until systems can be shut down safely.

C. Replication

Replication copies data or systems to another location or system. It supports disaster recovery and availability, but it does not keep the same data center powered during an outage.

D. Parallel processing

Parallel processing distributes computing tasks across multiple processors or systems. It does not provide backup power.

Therefore, a generator is the best answer for a multiday power outage.