The correct answer is B) The oxygen indicator measures the amount of oxygen in the atmosphere of a confined space. The oxygen indicator is used to monitor the level of oxygen in the atmosphere of a confined space, which is critical for ensuring the safety of personnel entering that space. It directly measures the percentage of oxygen in the air, not the concentration of other gases like CO2 or combustible gases. This allows the user to determine if the oxygen level is within a safe range to enter the confined space without risk of asphyxiation. The other answer choices are incorrect because they do not accurately describe the primary function of an oxygen indicator. It is not used to measure CO2 levels or the concentration of combustible gases, but rather the oxygen content of the atmosphere.