The analysis shows that ① n in ①ANEC is not axisymmetric;

② Neither K nor S in ②KBSM is axisymmetric;

③ Z in ③XIHZ is not axisymmetric;

④ ZZ in ZDWH is not axisymmetric;

So the two letters in (2) are not symmetrical, and there is only one other option.

Therefore, fill in ②.