The correct answer is LRU (Least Recently Used).
LRU is a page replacement policy that evicts the page that has not been used for the longest time. This policy is based on the assumption that pages that have not been used for a long time are less likely to be used in the future.
FIFO (First In First Out) is a page replacement policy that evicts the page that was first loaded into memory. This policy is simple to implement, but it is not very efficient.
LFU (Least Frequently Used) is a page replacement policy that evicts the page that has been used the least number of times. This policy is more efficient than FIFO, but it is not as efficient as LRU.
Working set is a set of pages that are currently in use by a process. The working set policy evicts pages from memory when the working set exceeds the available memory. This policy is efficient, but it is not very simple to implement.
None of the above is not a valid page replacement policy.