Which of these could not have occurred until the postclassical era

Postclassical era exist from 600 CE - 1450 CE

so, the one that could not have occurred until the postclassical era would be : West African Ruler converts to Islam
Because Islam started to exist/developed around the beginning of 600 CE.

hope this helps

Cherokee  removal, part of the trail of tears, refers to the  forced  relocation between 1836 and 1839 of thecherokee  nation from  their lands ingeorgia, south carolina, north carolina, tennessee, texas, and alabama to the indian territory (present day oklahoma)  in  the thenwestern united states, and the resultant deaths