In which one of the following page replacement algorithms it is possible for the page fault rate to increase even when the number of allocated frames increases?

The phenomenon where increasing the number of allocated page frames results in an increase in the number of page faults is known as Bélády's Anomaly.

• FIFO (First-In-First-Out) is susceptible to Bélády's anomaly because it is not a stack algorithm.

• LRU and OPT belong to the class of *stack algorithms*, for which the set of pages in memory for $n$ frames is always a subset of the pages in memory for $n+1$ frames, guaranteeing that Bélády's anomaly cannot occur.

Therefore, the correct option is (D) FIFO (First In First Out).