Here are the best answers for each question based on typical English grammar and usage:

1. The wedding party ____ held at the Rex Hotel.
- **B. is being**

2. How many languages ____ John speak?
- **A. does**

3. Hurry! The train ____ I don't want to miss it.
- **D. has come**

4. What time ____ the next train leave?
- **B. will**

5. Monica ____ with her sister at the moment until she finds a flat.
- **B. is staying**

6. He will take the dog out for a walk as soon as he ____ dinner.
- **C. finishes**

7. She ____ English at RMIT these days.
- **B. is studying**

8. She's at her best when she ____ big decisions.
- **C. will study**

9. We ____ next vacation in London.
- **A. spend**

10. Robert ____ tomorrow morning on the 10:30 train.
- **B. is arriving**

These answers fit the context and grammatical structure of each sentence.