Final answer:
The use of a buffer in a program accessing a single I/O device can reduce runtime significantly, potentially by more than a factor of two, as it allows for concurrent processing and reduces CPU idle times.
Step-by-step explanation:
When comparing unbuffered I/O with the use of a buffer in a program accessing a single I/O device, the use of a buffer can reduce the running time significantly, though the exact factor depends on the specific circumstances of the I/O operations and the system architecture. Buffers enable a program to run concurrently with some I/O operations, by storing or collecting data before the process actually requires it or sends it, thus reducing idle time waiting for these operations to complete.
The option that suggests a reduction by at least a factor of two is the minimum expectation because buffered I/O can smooth out the data transfer to and from the device, reducing the time the CPU spends waiting on I/O completion interrupts. However, in practice, the improvement in speed can be much greater than a factor of two, which corresponds to option (c): More than a factor of two.