Point Z is located at a high pH and it might be quick and easy to pick option (E) Metabolic alkalosis simply because of the high pH, but metabolic alkalosis would imply that the increase in pH is due to an increased bicarbonate concentration; point Z on the chart is indicated 20 mmol/L, a lower bicarbonate concentration.

(C) Breathing air at 2 atmospheres of pressure

This answer option is incorrect and discussed here on reddit. We typically breathe air at 1 atmosphere of pressure or 760 mm Hg. Due to Dalton's Law, if the pressure of the air increases, so too must the partial pressure of the constituent gases, which implies that there would be increased carbon dioxide concentrations. This would result in decreased blood pH due to respiratory acidosis.

(B) Breathing a gas mixture with a low PO_2

The chart has a high pH value -> alkalosis and a low CO2 pressure, and a low bicarbonate concentration -> respiratory alkalosis with renal compensation. The difficult part about this question is that respiratory alkalosis is usually indicated by a decrease in PCO2 but that's not immediately stated in any of the answers. One good example of breathing a gas mixture with a low PO_2 causing respiratory alkalosis is mountain sickness or altitude hypoxia. In low-pressure conditions, the peripheral chemoreceptors in the carotid body will respond to low partial pressures of oxygen by inducing hyperventilation and decreasing alveolar PCO2, raising the pH and resulting in respiratory alkalosis.