Answer:
(E) some species of cyanobacteria have specialized nitrogen-fixing cells
Step-by-step explanation:
The nitrogen is an essential element for life, it's included in the molecule of DNA and RNA. But, most of the nitrogen on the planet is in its inorganic gas form (N₂), which can't be captured for most organisms. But some prokaryotes can metabolic the N₂ to NH₃ in a process called nitrogen fixation. NH₃ can be used by some organisms, and it can be also transformed in nitrate, and then used by the organisms.
Only a few prokaryotes can do this: some bacterias that have a mutualistic symbiont process with plants, and some cyanobacteria of free-life. There is no specialized nitrogen-fixing cell, but there are specialized nitrogen-fixing enzymes.