Why: The main system board, which houses the CPU, memory, expansion slots, and other key components, is called the **motherboard**. It serves as the central circuit hub where all computer components connect and communicate. This is a fundamental part of computer organization and hardware structure. Option A matches this definition.

Why: The **CPU (Central Processing Unit)** is known as the brain of the computer because it handles all processing tasks, executes instructions, performs arithmetic and logical operations via ALU, and controls other components through the control unit. It is the core of computer organization. Option B is correct.

Why: The **processor (CPU)** directly executes program instructions by fetching them from memory, decoding them, and performing the required operations. It is the central component in computer system organization responsible for instruction execution. Other options like storage or peripherals do not execute instructions. Option D is correct.

Why: A computer follows the **fetch-decode-execute cycle**: First, it **fetches** the instruction from memory; then **decodes** it to understand the operation; finally, it **executes** the instruction. This is the standard instruction cycle in CPU organization. Option C matches this sequence.

Why: **Cache memory** is the high-speed memory that communicates most directly with the CPU, storing frequently used data for quick access and reducing latency compared to main memory (RAM). While registers are even closer, cache is the standard term for this direct communication in memory hierarchy. Option A is correct.