The correct answer is: A. 1
An IRQ, or Interrupt Request, is a signal that a device sends to the CPU to request attention. USB devices typically use a single IRQ, which is shared by all devices in the chain. This means that only one device can be active at a time, but this is usually not a problem as most devices only need to be active for short periods of time.
Option B is incorrect because it would require two IRQs, one for each device in the chain. This would be inefficient and could cause problems if two devices needed to be active at the same time.
Option C is incorrect because it would require three IRQs, one for each device in the chain plus one for the USB hub. This would be even more inefficient than Option B and would be very unlikely to be used in practice.
Option D is incorrect because it would require four IRQs, one for each device in the chain plus one for the USB hub plus one for the USB controller. This would be extremely inefficient and would be very unlikely to be used in practice.
Option E is incorrect because it is not a valid option.