The correct answer is A. CPU.
A CPU, or central processing unit, is the main processing unit of a computer. It is responsible for carrying out the instructions of a computer program. CPUs are typically the fastest components in a computer, and they are often the most expensive.
Magnetic tapes and disks are storage devices. They are used to store data, such as programs and files. Magnetic tapes are typically slower than magnetic disks, but they can store more data.
A video terminal is a device that displays text and images. It is typically used to interact with a computer. Video terminals are typically slower than CPUs, but they are often less expensive.
Sensors and mechanical controllers are devices that are used to control physical processes. They are typically slower than CPUs, but they are often more reliable.
In conclusion, the CPU is the fastest component in a computer. It is responsible for carrying out the instructions of a computer program.