The correct answer is C) Prolonged exposure to CO2 can result in false readings. The oxygen indicator is a critical instrument used to measure the oxygen content in confined spaces, which is important for determining the safety of the space before entry. Exposure to high levels of CO2 (carbon dioxide) can interfere with the oxygen indicator's ability to provide accurate readings, potentially leading to false results. This is because the oxygen indicator may interpret the presence of CO2 as a reduction in oxygen, even when the actual oxygen level is still within safe limits. The other answer choices are incorrect: A) Exposure to flue gas can affect the instrument's readings, B) the instrument should sample multiple levels of the tested space, and D) the oxygen indicator is not designed to detect hydrogen gas.