The correct answer is: A. IRQ
An IRQ (Interrupt Request) is a signal sent by a device to the CPU to request attention. If two devices are using the same IRQ, it can cause a conflict and the system may lock up.
A DMA (Direct Memory Access) channel is a way for a device to access memory directly, without going through the CPU. If two devices are using the same DMA channel, it can also cause a conflict.
A defective LPT port is a less likely cause, but it is possible. If the LPT port is not working properly, it can cause problems with any device that is connected to it.
A defective tape driver is also a less likely cause, but it is possible. If the tape driver is not working properly, it can cause problems with the tape backup unit.
A defective sound card is the least likely cause. If the sound card is defective, it is more likely to cause problems with sound playback or recording, rather than with the parallel port tape backup unit.
In conclusion, the most likely problem is that the sound card and the parallel port tape backup unit are both using the same IRQ. This can be resolved by changing the IRQ settings for one of the devices.