Which surface has wynne prakusya as the opponent in the final?
CREATE TABLE "singles_18_itf" (
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score" text
);
SELECT "surface" FROM "singles_18_itf" WHERE "opponent_in_the_final"='wynne prakusya';