Correct : d

Simulating page references with 3 frames:
Frames start empty. First 3 references fill frames — all are compulsory misses.
Ref 1: frames = {1, -, -} → miss
Ref 2: frames = {1, 2, -} → miss
Ref 3: frames = {1, 2, 3} → miss
Ref 4*: frames full → replace. frames = {1, 2, 4} or {4, 2, 3} or {1, 4, 3} → miss
Ref 5*: frames full → replace → miss
Ref 4: hit
Ref 1: hit or miss depending on what was replaced
Ref 6*: replace → miss
Ref 4: hit
Ref 5: hit or miss
Ref 1: hit
Ref 3*: replace → miss
Ref 2: hit or miss
Checking Optimal page replacement:
Optimal replaces the page that will not be used for the longest time in the future.
After {1,2,3} filled:
Ref 4*: future = 5,4,1,6,4,5,1,3,2. Next use: 1→pos 7, 2→pos 13, 3→pos 12. Replace 2 (used latest). frames = {1,3,4}
Ref 5*: future = 4,1,6,4,5,1,3,2. Next use: 1→pos 6, 3→pos 11, 4→pos 3. Replace 3 (used latest). frames = {1,4,5}
Ref 4: hit. frames = {1,4,5}
Ref 1: hit. frames = {1,4,5}
Ref 6*: future = 4,5,1,3,2. Next use: 1→pos 5, 4→pos 1, 5→pos 2. Replace 1 (used latest at pos 5). frames = {6,4,5}
Ref 4: hit
Ref 5: hit
Ref 1*: future = 3,2. Next use: 6→never, 4→never, 5→never. Replace any (say 6). frames = {1,4,5}
Ref 3*: future = 2. Next use: 1→never, 4→never, 5→never. Replace any (say 4). frames = {1,3,5}
Ref 2: miss or hit depending on above choices
The page replacements (*) at positions 4, 5, 8, 11, 12 match the Optimal policy behavior.
Option A (LRU) — Does NOT match. LRU would make different replacement choices that don't align with the given frame trace.
Option B (LFU) — Does NOT match. LFU tracks frequency counts and would replace differently.
Option C (MFU) — Does NOT match. MFU replaces the most frequently used page — leads to different replacements.
Option D (Optimal) — Matches. The given replacement pattern is consistent with Optimal policy which always replaces the page used farthest in the future.
∴ The page replacement policy in use is Optimal page replacement policy (Option D)