Here are the odd ones out for each set:

1. **C. by train** - The others are phrases that imply a method of occurrence or situation (by chance, by accident, by mistake), whereas "by train" is a specific mode of transportation.

2. **A. confide** - The others (donate, service) involve giving or providing in some manner, while "confide" involves sharing a personal secret.

3. **C. purpose** - The others (various, service) are adjectives describing nouns, while "purpose" is a noun itself.

4. **B. landslide** - The others (confident, provide) are traits or actions, while "landslide" refers to a specific event or phenomenon.

5. **C. mention** - The others (poverty, orphan) represent states or conditions, whereas "mention" is an action.