Bacteria which convert atmospheric nitrogen into nitrogen compounds are called
Correct option is A. Nitrogen-fixing bacteria
Plants cannot use the atmospheric nitrogen directly. Certain bacteria are involved in the conversion of nitrogen into nitrate compounds in the soil. Bacteria which convert atmospheric nitrogen into nitrogen compounds like ammonia are called nitrogen-fixing bacteria. These bacteria make nitrogen available for the plants. These are of 2 types, symbiotic nitrogen-fixing bacteria like Rhizobium and non-symbiotic nitrogen-fixing bacteria like Azotobacter.
Thus, the correct answer is 'Nitrogen-fixing bacteria.'