The answer is D.
The sequence is a simple alternating repetition of the letters U, B, I, P, W, D, F, Q, Z.
The first letter is U, the second letter is B, the third letter is I, the fourth letter is P, the fifth letter is W, the sixth letter is D, the seventh letter is F, the eighth letter is Q, and the ninth letter is Z.
The sequence repeats itself with each letter being two letters ahead of the previous letter.
For example, the first letter is U, which is two letters ahead of the letter B. The second letter is B, which is two letters ahead of the letter I. The third letter is I, which is two letters ahead of the letter P. The fourth letter is P, which is two letters ahead of the letter W. The fifth letter is W, which is two letters ahead of the letter D. The sixth letter is D, which is two letters ahead of the letter F. The seventh letter is F, which is two letters ahead of the letter Q. The eighth letter is Q, which is two letters ahead of the letter Z.
The missing letter is D.