CPU utilization is a primary indicator of how effectively your cluster's compute resources are being leveraged. A sustained utilization level between 70% and 80% typically signifies that executors are busy performing work without being starved or overloaded. This specific range is considered efficient because it maximizes throughput while leaving sufficient headroom to absorb unexpected spikes in demand without causing significant contention or queuing.

Why other options are incorrect:

• Load average: A steady five-minute load average does not reveal the actual percentage of CPU capacity being used; it can remain flat even if the cluster is under-utilized or unbalanced.
• Network I/O: Low network traffic might mean the job is not I/O-bound, but it could also indicate that data movement is under-utilized, which is not a definitive measure of overall resource efficiency.
• Disk usage: Disk capacity is not directly tied to runtime performance; unchanged disk usage simply means the job is not writing large amounts of data or spilling to disk, which does not guarantee that CPU and memory are being used effectively.