Four letter clusters have been given, out of which three are alike in some manner, while one is different. Choose the odd one.

Let us check the positional difference between consecutive letters in alphabetical order ($A=1,B=2,\dots ,Z=26$):RSU: $R\to S$ ($+1$)$S\to U$ ($+2$)YAC: $Y\to A$ ($+2$)$A\to C$ ($+2$)LNP: $L\to N$ ($+2$)$N\to P$ ($+2$)CEG: $C\to E$ ($+2$)$E\to G$ ($+2$)All clusters except RSU follow a $+2,+2$ incremental pattern.