Here are the appropriate answers along with explanations for each fill-in-the-blank sentence:

11. What is the **similarity** between a custom and a tradition? (SIMILAR)
- **Explanation**: A "similarity" refers to a point of resemblance or commonality between two things, in this case, customs and traditions.

12. In Viet Nam, we usually wait for the **oldest** person to sit down before you sit down. (OLD)
- **Explanation**: "Oldest" is the superlative form of "old," indicating the highest degree or most advanced in age.

13. Is he **permitted** to break the customs of her family? (PERMISSION)
- **Explanation**: "Permitted" is the past participle form of "permit," which indicates gaining permission to do something.

14. Today, we are going to discuss the **necessity** of traditions. (NECESSARY)
- **Explanation**: "Necessity" is a noun indicating the state of being necessary or required.

15. The offspring will follow the customs without **obligation**. (OBLIGE)
- **Explanation**: "Obligation" is a noun that refers to a duty or commitment, suggesting that following customs is done voluntarily in this context.

16. My dad is the **host** of this wedding anniversary party. (HOSTESS)
- **Explanation**: "Host" is the masculine form of "hostess," referring to the person who arranges and entertains guests.

17. Do you have to follow the traditions **strictly**? (STRICT)
- **Explanation**: "Strictly" is an adverb that means in a strict manner, often indicating that there are no exceptions.

18. She was taught a lot **of** skills by my parents. (SOCIETY)
- **Explanation**: "Societal" skills would refer to social skills, however, "of" fits better in this context, as the intended noun is not necessarily provided.

19. In some ethnic groups, the elderly **oblige** their next generations to accept the customs. (OBLIGATION)
- **Explanation**: "Oblige" (verb) refers to making someone do something, often perceived as a duty or requirement.

20. We were impressed by the **beauty** of Ha Long Bay. (BEAUTIFUL)
- **Explanation**: "Beauty" is the noun form describing the aesthetic value of Ha Long Bay.

If you need further explanations or clarifications on any of the answers, feel free to ask!