Which gene cluster is responsible for nitrogen fixation in bacteria?

The correct option is B Nod, Nif, Fix

Nitrogen fixation is a process by which nitrogen present in the atmosphere is converted into ammonia. It is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing compounds. This process is done with the help of nitrogen-fixing bacteria that have Nod, Nif and Fix gene clusters responsible for nitrogen fixation.

So the correct answer is 'Nod, Nif, Fix'.