Let us each option one by one.
Option A) is true because threads share common data , memory and code segments , so proper synchronisation mechanism should be provided to enhance the security of the system
Option B) is true because the kernel level threads are heavier as they have more context hence the context switch overhead will increase making the kernel level threads slower as compared to user level threads.
Option C) is false because the OS views the user level threads as process only so even though the threading support is not there , user threads will not be affected.
Option D) is true since we know that blocking the kernel thread only blocks that corresponding thread but blocking user level thread blocks the entire process since as mentioned earlier the OS views the user level threads as processes only.So if an application that gets blocked frequently , for them kernel level thread is better than user level thread.
Hence option C) should be the correct option.
