If you want to use hints you should use /*+...*/
as below
SELECT /*+ FULL( fst) full(fs)*/ COUNT(DISTINCT r1.CLIENT_ID || e.CODE || fst.TAB_ID) NB_TAB
FROM RATTACHEMENT_DART rd, FLUX_SORTANT fs,
FS_TABLEAU fst, equipement e,
REFERENTIEL r1, referentiel r2, referentiel pere;