Nitrogen-fixing bacteria get nitrogen from: the soil the atmosphere plant protein legumes

The plants absorb the nitrogen from the atmosphere and it transports down into the soil where the nitrogen-fixed bactera changes the form of the nitogen:)

D, Legumes

Explanation:

In a symbiotic relationship with the soil bacteria known as 'rhizobia', legumes form nodules on their roots (or stems, see figure below) to 'fix' nitrogen into a form usable by plants (and animals). ... Plants cannot fix nitrogen on their own, but need it in one form or another to make amino acids and proteins.