Final answer:
The primary benefit of making a computing system fault-tolerant is enhanced reliability. It ensures the system continues to operate even when parts fail, which is key for mission-critical applications.
Step-by-step explanation:
The primary benefit of making a computing system fault-tolerant is C) Enhanced reliability. Fault-tolerance is designed to ensure a system remains functional even in the event of a partial system failure. This is achieved through redundancy and various failover mechanisms. It does not inherently increase the speed of a system (A), nor does it necessarily improve security (B) or reduce costs (D), as redundancy and handling faults can increase the complexity and expense of a system. Instead, the advantage lies in the system's continued operation despite failures, which is crucial for mission-critical applications where downtime can have significant negative consequences.