The correct answer is A. CPU. The CPU, or central processing unit, is the processor that controls and directs computer activities. The operating system provides instructions, and the CPU processes them. An HDD stores data, RAM temporarily stores active data and instructions, and a NIC provides network connectivity. Only the CPU executes instructions.
Exact Extract/Reference: The CompTIA Tech+ Study Guide states that the role of the CPU is "to control and direct all the activities of the computer" and that the terms "processor" and "CPU" are interchangeable. It also explains that the processor "just does what it's instructed to do."