1. Tet is a Vietnamese tradition that is an occasion for the family **reunion**.
- **Explanation**: Tet, also known as the Lunar New Year, is a time when family members gather together to celebrate and spend time with each other. The word "reunion" fits best in this context as it refers to the act of coming together again after being apart.

2. The three-year-old boy grew up very fast and **defeated** invaders with only bamboo trees.
- **Explanation**: The word "defeated" means to overcome or beat an opponent, which fits the context of a boy overcoming invaders. The other options do not fit the context of overcoming or winning against invaders.

3. The **ritual** that is carried out first in most festivals in Viet Nam often begins with an offering of incense.
- **Explanation**: A "ritual" refers to a series of actions or type of behavior regularly and invariably followed by someone, especially as part of a ceremony. In Vietnamese festivals, rituals often include the offering of incense as a traditional practice.