Which organisms are microaerobic, free fixers or in association with grasses

Answer: Option B

Azospirillum is a microaerobic organism, that free fixers or survives in association with grasses.
These are gram-negative, microaerophilic, non-fermentative, and nitrogen-fixing bacterial genus from the family of Rhodospitillaceae.
This bacteria helps in promoting plant growth by mechanisms of tolerance of abiotic stresses named as induced systemic tolerance, mediated by antioxidants, osmotic adjustment, production of phytohormones, and defense strategies.