Introduction:
A hardware interrupt is a signal generated by a hardware device to interrupt the normal execution of a program and transfer control to a specific interrupt handler routine. It allows the hardware device to communicate with the processor, informing it of events that require immediate attention.

Explanation:
A hardware interrupt can be either internal or external, depending on the source of the interrupt. Let's understand each option in detail:

a) Also called an internal interrupt:
- An internal interrupt, also known as a software interrupt, is generated by the CPU itself.
- It is triggered by a special instruction in the program code that causes the CPU to transfer control to a specific interrupt handler routine.
- Internal interrupts are not directly related to hardware devices, so they do not fall under the category of hardware interrupts.

b) Also called an external interrupt:
- An external interrupt is generated by an external hardware device.
- When a hardware device needs to interrupt the normal execution of a program, it sends a signal to the processor, indicating that an interrupt has occurred.
- The processor then transfers control to the corresponding interrupt handler routine to handle the event.
- External interrupts are directly related to hardware devices and are commonly used for input/output operations, such as keyboard input, mouse input, etc.

c) An I/O interrupt:
- An I/O interrupt is a specific type of hardware interrupt that is generated by input/output devices.
- When an input/output device, such as a disk drive or a network card, completes a task or encounters an error, it can generate an I/O interrupt to inform the processor.
- The processor then interrupts the current program execution and transfers control to the appropriate interrupt handler routine to handle the I/O operation.

d) A clock interrupt:
- A clock interrupt is a specific type of hardware interrupt generated by the system clock or a timer device.
- It is used to keep track of time and schedule tasks.
- The clock interrupt occurs at regular intervals and is used by the operating system to perform various time-related operations, such as updating the system time, scheduling tasks, etc.

Conclusion:
In conclusion, a hardware interrupt is a signal generated by a hardware device to interrupt the normal execution of a program. An external interrupt, also known as a hardware interrupt, is triggered by an external hardware device and is used for input/output operations. Therefore, the correct answer is option 'B' - Also called an external interrupt.