In A.b.c.d.e.f.g.h.i.j.k.l.m.n.0.p.q.r.s.t.u.v.w.x.y.z what letter is missing?

The numeral zero (0) is used in place of the lowercase letter O (o).

In most fonts, the 0 is taller than the (o) and narrower than the capital O.