The Achaemenids fell due to a combination of

AA weakening army and succession problems
BGrowing intolerance of other cultures and internal rebellions
CAttacks from central Asia and weak bureaucracy
DBureaucratic infighting and a discontented population

Verified Answer :

The Achaemenids fell due to a combination of "b. Growing intolerance of other cultures and internal rebellions," since this led to people feeling as though they were not being respected by their government.