The first, fourth and seventh letters are in alphabetical order. So, tenth letter would be the letter after C i.e. D.
Also the second and third letter are alternate and in reverse order and so are the fifth and sixth letters and the eighth and ninth letters.
The given sequence consists of two series.
I. Z, W, T, Q, ? in which each letter is moved three steps backward to obtain the next term.
II. S, O, K, G in which each letter is moved four steps backward to obtain the next term.