Closure [$S' \rightarrow .S$] =
$S' \rightarrow .S$
$S \rightarrow X$
$X \rightarrow .YX$
$X \rightarrow .$
$Y \rightarrow .aY$
$Y \rightarrow .b$
There are two SR conflicts(shift on $a, b$ & reduce on $X \rightarrow .$ in this $LR(0) $ item.
Hence it is not LR(0) grammar