Which of the following elements is fixed by cyanobacteria?

Cyanobacteria are remarkable prokaryotes primarily known for their ability to perform nitrogen fixation. This vital biological process converts atmospheric nitrogen (N2), which is unusable by most organisms, into ammonia (NH3) and other nitrogenous compounds essential for life. This makes nitrogen available for plants and other organisms, playing a crucial role in the global nitrogen cycle.

• Oxygen (A) is produced by cyanobacteria through photosynthesis, not fixed.
• Carbon (B) is fixed by cyanobacteria as carbon dioxide during photosynthesis, but nitrogen fixation is their most distinctive elemental fixation process.
• Hydrogen (D) is an element but not directly "fixed" in the same metabolic context as nitrogen or carbon by cyanobacteria.