Which of the following is not a valid deadlock prevention scheme?
The answer is (C).
Actually, C increases the chances of deadlock. You are not allowed to request resources after releasing some resources. So if you require to request some resources and for some of the other resources you have finished your work, What will you do?
You will not release those for which you have finished work and request those you required. So you are holding more number of resources which might be useful for some other process. Increase the chance of deadlock.