The correct answer is Registers - they are the most immediate form of storage and located directly in the CPU. Registers are the fastest and smallest types of storage and hold all data being processed in real time within the CPU (central processing unit).
Cache is high speed memory, but it is not on the CPU. Therefore, cache can be confused with Registers but should not be compared because Cache contains different types of memory with different speeds corresponding to L1/L2/L3 location near the CPU which helps to facilitate faster access to RAM.
RAM is main memory (larger than either cache or registers), but it is also slower than either; it exists off chip from the CPU.