Nitrox and the acronym EANx refer to mixtures with oxygen concentrations (FO2) higher than:

Enriched Air Nitrox, commonly abbreviated as EANx, refers to air mixtures that have oxygen concentrations higher than the standard atmospheric level of about 21%. When divers use Nitrox, they typically do so to take advantage of the increased oxygen content, which allows for longer bottom times and shorter surface intervals compared to diving with regular air.

The significance of the 21% mark lies in its representation of the normal oxygen level in air; therefore, any mixture that contains a higher percentage qualifies as 'enriched air' or Nitrox. This also aligns with general diving practices where nitrogen's effects on the body are mitigated by elevating the oxygen content within the gas mix.

The other percentages listed, while relevant in diving practice, each represent different gas mixtures or considerations but do not define the threshold for Nitrox. Therefore, the correct answer reflects the baseline standard for distinguishing between regular air and enriched air mixtures used in diving scenarios.