The operation executed on data stored in registers is called

Computer architecture classifies operations on register data along two independent axes: (1) data width -- how many bits are acted on at once (a single bit, a byte, a word) -- and (2) execution level -- whether the action is a single, indivisible hardware step carried out in one clock cycle, or a composite action assembled from a sequence of such steps to realize one full machine instruction.

The stem asks for the term describing an operation executed directly on data already held in registers -- that is a question about execution level, not data width. The term for a single, indivisible hardware-level action on register contents is a micro-operation, so the correct term is Micro-operation.

Why the other options do not fit:

• Macro-operation: sits at the opposite end of the execution-level axis -- it is the composite, multi-step machine instruction (e.g. ADD, LOAD, MOV) assembled from several micro-operations, not a single elementary action.

• Bit-operation: this names a point on the data-width axis (one bit), unrelated to the execution-level axis the stem is asking about.

• Byte-operation: also a data-width term (eight bits, one byte); like bit-operation, it says nothing about whether the action is elementary or composite.

Cross-check: the stems phrase, operation executed on data stored in registers, asks specifically about execution level (elementary versus composite), which only the term micro-operation addresses directly among the four options; the other three either sit at the composite end of that axis or belong to the unrelated data-width axis entirely.