The odd one out in this group is option 'D', which is "Seldom".

Explanation:

Besides, However, Moreover, and Seldom are all transitional words that are used to connect ideas or provide additional information in a sentence. However, 'Seldom' is the odd one out because it is not commonly used as a transitional word like the other options.

1. Besides: This transitional word is used to add information or provide an alternative perspective to the main idea. It is often used to introduce an additional point or reason. For example: "Besides, it was raining heavily, so we decided to stay indoors."

2. However: This transitional word is used to show a contrast or contradiction to the previous statement. It is often used to introduce a different viewpoint or present an unexpected outcome. For example: "I wanted to go to the party; however, I had to stay home and study for my exams."

3. Moreover: This transitional word is used to add more information or evidence to support the main idea. It is often used to introduce an additional point or highlight a significant aspect. For example: "She is not only intelligent but moreover, she has a great sense of humor."

4. Seldom: Unlike the other options, 'Seldom' is not commonly used as a transitional word. It is an adverb that means "rarely" or "not often." While it can be used to provide additional information in a sentence, it does not serve the same function as the other transitional words in this group.

In conclusion, the odd one out in this group is 'Seldom' because it is not commonly used as a transitional word like the other options.