When the cache space fills up, I/O will be queued until an empty cache slot is available. This is because PowerMax operates as a cache-centric architecture, where all data is passed through cache before being stored on disk. Cache is used to buffer incoming and outgoing data, as well as to support various functions such as replication and data reduction. When cache is full, the array cannot accept any more data until some cache slots are freed up by flushing data to disk or invalidating stale data. Therefore, answer B is correct.

A, C, and D are incorrect because they do not describe what happens when cache space fills up. I/O will not be serviced at the speed of the destination device (A), as this would bypass the cache and degrade performance. Cache is not permanently flushed © or temporarily disabled (D), as this would result in data loss or corruption.