Which of the following is tier IV for data center design according to "Data Center Site Infrastructure Tier Standard: Topology"?

The tier IV designation for data center design, as outlined in the "Data Center Site Infrastructure Tier Standard: Topology," is characterized by fault-tolerant site infrastructure. This means that the data center is designed to provide the highest level of availability and reliability. Tier IV facilities are capable of sustaining operations even in the event of a failure of any single component in the system.

These data centers have redundant systems, allowing for maintenance without downtime and ensuring that any issues can be resolved without affecting operational continuity. The fault-tolerant infrastructure includes multiple independent paths for power and cooling, which are crucial for maintaining full operation during expected failures, thereby achieving optimal resilience.

In contrast, concurrently maintainable site infrastructure pertains to tier III, which ensures systems remain operational during maintenance but does not require complete fault tolerance. Redundant site infrastructure capacity components is a concept found in lower tiers that involve redundancy but not to the extent that guarantees fault tolerance. Basic data center site infrastructure represents tier I, which lacks redundancy and is susceptible to downtime during any maintenance or component failure. Thus, the distinguishing feature of tier IV is indeed its fault-tolerant infrastructure, making it the correct answer.