Explanation:

- Cache: A small-sized type of volatile computer memory that provides high-speed data access to the processor. It sits between the CPU registers and RAM in the memory hierarchy.

- RAM (Random Access Memory): Main memory, where data is held temporarily for quick access by the processor during tasks. Faster than secondary memory but slower than cache.

- Secondary memory: Refers to storage devices like hard drives and SSDs. They are non-volatile and used to store data long-term. Access speed is slower compared to cache and RAM.

- CPU registers: Small storage locations within the CPU used for storing instructions or data immediately needed by the CPU. They are the quickest to access.

Among these, the slowest in the hierarchy is:

Option:3 - Secondary memory