Explanation:

In this question, we need to find the odd letter cluster among the given options. Let's analyze each option one by one to determine the odd one out.

a) NM:
The letter cluster "NM" does not have any unique characteristic that sets it apart from the other options. Therefore, it is not the odd cluster.

b) BD:
The letter cluster "BD" is the odd one out because it does not follow the pattern of the other options. In options a), c), and d), the first letter is the successor of the second letter in the English alphabet. However, in option b), the first letter "B" is the predecessor of the second letter "D" in the English alphabet. Therefore, option b) is the correct answer.

c) IR:
The letter cluster "IR" follows the pattern of the other options, where the first letter is the successor of the second letter in the English alphabet. Therefore, it is not the odd cluster.

d) PK:
The letter cluster "PK" follows the pattern of the other options, where the first letter is the successor of the second letter in the English alphabet. Therefore, it is not the odd cluster.

Summary:

- Option a) NM does not have any unique characteristic that sets it apart from the other options.
- Option b) BD is the odd one out because the first letter "B" is the predecessor of the second letter "D" in the English alphabet.
- Option c) IR follows the pattern of the other options.
- Option d) PK follows the pattern of the other options.

Therefore, the correct answer is option b) BD.