Answer is: b. air.
Homogeneous mixture (air) has the same proporties of its components (nitrogen, oxygen, carbon dioxide) throughout any given sample.
Together, nitrogen (N₂) and oxygen (O₂) by volume account for 99,03% of the gases composing the dry air in the lower atmosphere.
Volume of nitrogen in air is 78,08%.
Volume of oxygen in air is 20,95%.
Together: 78,08% + 20,95% = 99,03%.