Cache is the fastest kind of computer memory here. It’s super quick and sits close to the CPU to speed up how fast the computer grabs info. It keeps the stuff you use a lot handy, so it doesn't have to go all the way to the regular memory (RAM) every time.
Flash is slower and keeps data even when the power is off.
Hard disk is even slower and stores things for a long time.
ROM is only for reading and is slower than both RAM and cache; it's where the firmware lives.
Quick Info: If "Register" were an option, it would be the fastest, but among the choices provided, Cache is the correct answer.