C. Installing redundant PSUs (Power Supply Units):

Redundant PSUs provide fault tolerance by ensuring that if one power supply fails, another will take over seamlessly.

This is a cost-effective way to increase uptime while waiting for the UPSs to arrive.

Incorrect Options:

A. Purchasing backup generators: Generators are costly and are typically used for long-term outages, not as an immediate or cost-effective solution.

B. Increasing input voltage: This is not a practical solution to hardware failures or power issues.

D. Activating a hot site: A hot site is an expensive, fully operational backup facility that would not be a cost-effective solution for this scenario.

Key Takeaway: Installing redundant PSUs is a cost-effective way to balance uptime requirements in case of power or hardware failure.