Group by symmetry type: Letters with vertical symmetry: A, M, U, W, H (multiple axes). Letters with horizontal symmetry: B, D, E. Letters with no symmetry: N.
Grouping: (D, H, M) like symmetrical, (E, U, A), (N, W, B). Among options, (a,d,g);(b,e,h);(c,f,i) best groups: (D,U,B), (E,W,M), (N,H,A).