The ALU of a computer responds to the commands coming from the

The ALU acts on the commands sent by the control section of the processor.

Primary memory stores data and instructions but does not command the ALU.

External memory is meant for long-term storage, not for directing the ALU.

Cache memory speeds up access but does not issue the ALU's commands.

The control unit decodes instructions and tells the ALU what to do.

Together the ALU and control unit form the core of the CPU.