LIFE SCIENCE

Detailed explanation-1: -All known organisms which involve in nitrogen-fixing which are called diazatrophs are prokaryotes, and both bacterial and archaeal domains are responsible for that. Diazotrophs are categorized into two main groups namely: root-nodule bacteria and plant growth-promoting rhizobacteria.

Detailed explanation-2: -Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms that do this are rhizobia and Frankia (in symbiosis) and Azospirillum.

Detailed explanation-3: -Some diazotrophs are free-living whereas others are in symbiotic relationship with other organisms. Examples of free-living diazotrophs are as follows: anaerobic diazotrophs such as Clostridium spp.

Detailed explanation-4: -So the correct option is ‘four’.