Instruction Code
An instruction code is a group of bits that tells the computer what specific operation to perform. It serves as a command to the computer's central processing unit (CPU) to execute a specific task. The instruction code is an integral part of machine language, which is the low-level programming language understood by the computer's hardware.

Micro-Operation
A micro-operation refers to the smallest operation performed by a computer's CPU. It involves the manipulation of data within the CPU registers and is typically executed in a single clock cycle. Micro-operations are fundamental building blocks for executing complex instructions.

Accumulator
An accumulator is a register in a computer's CPU that is used for storing intermediate results and performing arithmetic and logical operations. It is a special-purpose register that plays a crucial role in many computer architectures. The accumulator can store data temporarily while calculations are being performed, and it can also serve as the destination for the result of an operation.

Register
A register is a small unit of high-speed memory located within the CPU. It is used for storing data that is frequently accessed by the CPU during its operations. Registers are used to hold operands, intermediate results, and memory addresses. They provide fast access to data, allowing the CPU to perform calculations and operations quickly.

Explanation
Among the given options, the correct answer is option 'A' - Instruction code. The instruction code is a group of bits that tells the computer what specific operation to perform. It is essentially a command that directs the CPU to execute a particular task. The instruction code is an essential part of machine language, which is the language understood by the computer's hardware.

Micro-operations, on the other hand, refer to the smallest operations performed by the CPU. These operations involve the manipulation of data within the CPU registers and are executed in a single clock cycle.

The accumulator is a special-purpose register that is used for storing intermediate results and performing arithmetic and logical operations. It plays a crucial role in many computer architectures.

Registers, in general, are small units of high-speed memory located within the CPU. They are used for storing frequently accessed data during the CPU's operations. Registers provide fast access to data, enabling the CPU to perform calculations and operations quickly.

In summary, while micro-operations, the accumulator, and registers are all important components of a computer's architecture, the group of bits that specifically tells the computer to perform a specific operation is known as the instruction code.