Reference String: $1,2,4,5,2,1,2,4$
Frame can contain only $3$ pages, $1$ and $2$ are already present
$1$: Hit, Frame Status: $1,2$
$2$: Hit, Frame Status: $1,2$
$4$: Page fault, Frame Status: $1,2,4$
$5$: Page fault, $1$ will be replaced as per LRU policy, Frame Status: $5,2,4$
$2$: Hit, Frame Status: $5,2,4$
$1$: Page fault, $4$ will be replaced, Frame Status: $5,2,1$
$2$: Hit, Frame Status: $5,2,1$
$4$: Page fault, $5$ will be replaced, Frame Status: $4,2,1$
4 Page faults hence option B) is correct