The correct option is (b) affixed.bs.affix
To explain: affixed.bs.affix occurs after fixed positioning is added to the element e.g. after .affix-top class is replaced with .affix class, affix-bs.affix occurs before fixed positioning is added to the element e.g. when .affix-top class is to be replaced with .affix class. Affix-top-bs.affix occurs before top ekement return to its original place. Affixed-top-bs.affix occurs before top ekement return to its original place.