What happens to the nitrogen at W?

Question

a
Its unusable form is converted into a form that is usable by bacteria in the soil.
b
Its unusable form is directly transferred up the tree roots.
c
Its usable form is inhaled and turned into proteins by the rabbit.
d
Its usable form is bonded with other molecules in the atmosphere.

Answer 1

a
Its unusable form is converted into a form that is usable by bacteria in the soil.