To find the correct letter sequence in the given series, we need to identify the pattern based on the English alphabetical order.

Let's examine the series: END, BRA, YVX, VZU, ?

1. For "END", the letters are E, N, and D. These have alphabetical positions 5, 14, and 4, respectively.
2. For "BRA", the letters are B, R, and A. These have positions 2, 18, and 1, respectively.
3. For "YVX", the letters are Y, V, and X. These have positions 25, 22, and 24, respectively.
4. For "VZU", the letters are V, Z, and U. These have positions 22, 26, and 21, respectively.

To identify the pattern, observe the following:

• The first letters (E, B, Y, V) change by -3, +23, -3, respectively.
• The second letters (N, R, V, Z) change by +4, +4, +4, respectively.
• The third letters (D, A, X, U) change by -3, +23, -3, respectively.

Continuing this pattern:

• The next first letter after V (position 22) is 22 - 3 = 19. The 19th letter is S.
• The next second letter after Z (position 26) is 26 + 4 = 30. As there are only 26 letters, wrap around to position 4, which is D.
• The next third letter after U (position 21) is 21 - 3 = 18. The 18th letter is R.

Hence, the next sequence in the series is "SDR".

Therefore, the correct answer is SDR.