Hierarchical memory organization
I get everything about this picture perfectly clear, but what I don't understand is where is ROM located? Isn't ROM part of BIOS? Where do we sort BIOS in this memory organization?


Nowhere. It's not part of this hierarchy at all.

The memory hierarchy shown here is for the types of storage that are routinely used to store and retrieve data while a computer is running. The BIOS ROM is not used this way -- it is read from briefly when the computer is booted up, and is only written to as a rare special case during updates.