Here are the best matches for the indefinite articles in Spanish:
- Posters - B. Unos (Posters is plural and masculine, so 'unos' is appropriate)
- Chair - B. Una (Chair is singular and feminine, so 'una' is appropriate)
- Windows - C. Unas (Windows is plural and feminine, so 'unas' is appropriate)
So the answers are:
- B
- B
- C