What letter should replace the question mark? x, u, r, o, ?

C. N

Complete the series: z, x, v, t, ?

A. R

Identify the missing letter: a, e, i, o, ?

A. U

Complete the series: a, c, e, g, ?

B. H

Find the missing letter in the series: b, d, f, h, ?

A. J