Answer is ST, ND, RD, TH, "TH".
The logic behind is: Displayed the series of Ordinal number suffixes.
Ordinal number series of {1,2,3,4,5} is {First, Second, Third, Fourth, Fifth }
{First, Second, Third, Fourth, Fifth } equals { 1st,2nd,3rd,4th,5th}
Suffixes of {1st,2nd,3rd,4th,5th} is {st,nd,rd,th,th}
