The correct option is A oxygen scavenger
Leghaemoglobin is an oxygen carrier protein which is found in the nitrogen-fixing root nodules of a leguminous plant. It is produced by the legumes in response to roots being colonized by nitrogen-fixing bacteria as a part of symbiotic interaction between plant and bacterium.
Nitrogenase enzyme is highly sensitive to molecular oxygen and becomes non-operational in aerobic conditions. Leghaemoglobin protects this enzyme by providing it with anaerobic conditions to function properly.