The primary unit of data storage in the personal computer is the

The correct answer is b. hard disk drive.

Explanation: The hard disk drive (HDD) is the primary unit of data storage in a personal computer, where the operating system, applications, and files are kept permanently. While RAM (option c) is important for temporary data access and speed, it is volatile and does not store data long-term. External cache is a small amount of memory used for speeding up data access and media cards are usually used for additional storage in devices like cameras or smartphones, rather than as a primary storage unit for PCs.