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The substance that acts as the H2O absorber in a pulmonary function testing machine is calcium sulfate. Calcium sulfate is commonly used in various applications including as a desiccant due to its ability to absorb moisture effectively. In the context of pulmonary function testing, it is crucial to remove water vapor from the gas sample to ensure accurate measurement of other gases such as oxygen and carbon dioxide.

While silica gel is also known as a drying agent and can absorb moisture, it is less common in this specific application compared to calcium sulfate. Sodium chloride, on the other hand, is more associated with the maintenance of osmotic balance and generally does not serve as an H2O absorber in pulmonary function testing. Activated charcoal is mainly used for adsorbing organic vapors and gases rather than moisture, making it unsuitable for this purpose. Thus, calcium sulfate is the preferred choice for effective water absorption in pulmonary function testing machines.