Final answer:
The least verbose logging level in Cassandra is ERROR, which reports only the most severe problems that may demand immediate action, unlike TRACE or DEBUG that provide more detailed information for debugging purposes.
Step-by-step explanation:
The least verbose logging level in Cassandra is d) ERROR. Logging levels are used to control the amount of information that is output by the system, and they typically range from most to least verbose as follows: TRACE, DEBUG, INFO, WARN, and ERROR. TRACE provides the finest granularity of information for debugging, while ERROR reports only the most severe problems that may require immediate attention.