The correct answer is: C. single bus structure.
A single bus structure is a type of computer bus in which all devices are connected to a single shared bus. This is the most common type of bus structure, and it is relatively simple and inexpensive to implement. However, it can be a bottleneck for performance, as all devices must share the same bandwidth.
A star bus structure is a type of computer bus in which each device is connected to a central hub. This can improve performance, as each device has its own dedicated bandwidth. However, it can be more expensive to implement, as the hub is required.
A multiple bus structure is a type of computer bus in which there are multiple buses, each of which is connected to a different set of devices. This can improve performance, as devices can communicate with each other on different buses without interfering with each other. However, it can be more complex and expensive to implement.
A node to node bus structure is a type of computer bus in which each device is connected directly to every other device. This can provide the best performance, as there is no shared bandwidth. However, it can be the most complex and expensive to implement.
In conclusion, the usual BUS structure used to connect the I/O devices is a single bus structure. This is because it is the most common type of bus structure, and it is relatively simple and inexpensive to implement.