Final answer:
The offset field in a memory address is used to pinpoint the exact location of data within a cache block after it has been retrieved by a CPU.
Step-by-step explanation:
The offset field of a main memory address is used to determine C. the location of the desired data within the cache block. When a CPU accesses memory and receives a cache block, the offset specifies the exact location of the data within that block. The offset is part of a larger address structure that typically includes a tag, an index, and the offset itself. To illustrate, imagine a memory system where each cache block accommodates 32 bytes of data; the offset field for this system would specify which one of those 32 bytes is needed.