Oxygen is harmful for (b) nitrogen fixing bacteria.
Explanation: Nitrogen-fixing bacteria require anaerobic conditions (without oxygen) to function effectively, so the presence of oxygen can actually inhibit their ability to fix nitrogen.
Key points about the other options:
Ferns, chara, and mango trees: These are all plants that need oxygen for respiration and photosynthesis.
Nitrogen fixation: This is the process by which certain bacteria convert atmospheric nitrogen into a usable form for plants.