Missing Letter: DUC _

The missing letter in the given sequence "DUC _" can be found by analyzing the pattern or rule followed by the letters. Let's break down the options and find the correct answer.

a) K
b) C
c) U
d) S

Analyzing the Pattern:
To find the missing letter in the sequence, we need to look for a pattern or rule that is followed by the given letters. Let's examine each option to determine which one fits the pattern.

1. Option a) K:
If we look at the sequence "DUC _", we can observe that the missing letter should come after the letter "C." Considering the alphabetical order, the next letter after "C" is "D." However, "K" does not follow the pattern of the sequence. Therefore, option a) is not the correct answer.

2. Option b) C:
In this option, the missing letter is the same as the last letter in the given sequence. However, this does not follow the pattern observed in the sequence "DUC _." Therefore, option b) is not the correct answer.

3. Option c) U:
In this option, the missing letter is the same as the second letter in the given sequence. The pattern observed in the sequence "DUC _" suggests that the missing letter should be the second letter, "U." This follows the rule of the sequence. Therefore, option c) is the correct answer.

4. Option d) S:
In this option, the missing letter is not consistent with the pattern observed in the sequence "DUC _." Therefore, option d) is not the correct answer.

Conclusion:
Analyzing the given sequence "DUC _," the missing letter can be determined by observing the pattern. The correct answer is option c) U.