Here are the best choices to fill in the blanks:

1. **B. joined** - "Last Sunday Nam and many of his friends (1) joined a book fair in his neighbourhood."
2. **A. to** - "The book fair is an annual charitable activity with a view to collecting money and used books, and donating them (2) to poor students in his community."
3. **C. were** - "There (3) were also some interesting competitions in the book fair, such as answering quizzes about famous books, composing poems, solving secret coded messages, etc."
4. **B. winners** - "The (4) winners received various gifts like handmade cards, cute teddy bears or school things."
5. **A. but** - "Unluckily, Nam didn’t win any games, (5) but he still felt very happy."

So the final text will read:

Last Sunday Nam and many of his friends (1) **joined** a book fair in his neighbourhood. The book fair is an annual charitable activity with a view to collecting money and used books, and donating them (2) **to** poor students in his community. Like other friends, Nam donated some used student’s books and bought some English short stories, comics and Vietnamese novels. He hoped he could make a small contribution to his community by doing so. There (3) **were** also some interesting competitions in the book fair, such as answering quizzes about famous books, composing poems, solving secret coded messages, etc. The (4) **winners** received various gifts like handmade cards, cute teddy bears or school things. Unluckily, Nam didn’t win any games, (5) **but** he still felt very happy.