In _________ page replacement algorithm, the page that will not be used for…

Concept — every page-replacement policy is defined by a single eviction key: at a page fault the policy computes one value for each page currently resident in a frame and discards the page whose value is extreme. The classical policies differ only in what that value is measured from — the access history already recorded, the order in which pages arrived, an access count, or the sequence of references still to come.

Applying it here — the blank must be filled by the policy whose eviction key is "how far ahead this page is next referenced". Discarding the resident page whose next use lies farthest ahead — that is, the page that will not be used for the longest period of time — is precisely the definition of the Optimal page-replacement algorithm, also written OPT, MIN, or Belady’s algorithm. So the blank is Optimal.

Contrast — the same four eviction keys separate the classical policies:

• largest time elapsed since the last reference (a backward-looking key) → LRU

• earliest load time, i.e. arrival order, with reference activity ignored → FIFO

• smallest number of accesses so far (a frequency key) → LFU

• greatest distance to the next reference (a forward-looking key) → Optimal

Of these four keys, the forward-looking one is the one that uses information which has not happened yet, and the stem states its rule as something that "will not" happen — a claim about what is still to come.

Why no reference string is needed — this item asks for the name of the policy whose rule is quoted, not for a page-fault count, so no concrete reference string is required; a reference string would be needed only to simulate the policy and count faults. Indeed, the fact that a running system cannot see the future is exactly why Optimal cannot be implemented online: it is evaluated offline on a recorded reference trace and used as a lower bound — the fewest page faults any policy could possibly incur — against which LRU, FIFO and LFU are measured.