Final answer:
Multithreading on a multi-CPU machine can increase or decrease the concurrency depending on the specific situation.
Step-by-step explanation:
The correct answer to the question is d) Can increase or decrease the concurrency. On a multi-CPU machine, using multithreading can have different effects on concurrency depending on the specific situation. It is possible for multithreading to increase concurrency by allowing multiple threads to run concurrently on different CPUs, which can improve performance. However, it is also possible for multithreading to decrease concurrency if threads compete for shared resources and introduce synchronization overhead. Therefore, the effect of multithreading on concurrency can vary.