The correct answer is A) To calibrate a gas analyzer to ensure it indicates correct gas concentrations. Span gas is used to calibrate the span or upper end of a gas analyzer's measurement range. By introducing a known concentration of the target gas, the analyzer can be adjusted to accurately read that concentration, ensuring the instrument provides reliable measurements across its full operating range. The other options are incorrect because: B) is for the zero point, not the span; C) describes the boiling point, not a calibration gas; and D) refers to an inert atmosphere, not span gas.