The correct answer is A) fresh air. After each reading of an oxygen indicator, the instrument should be purged with fresh air to ensure accurate and reliable measurements. This is a standard practice in accordance with the regulations and guidelines for the use of oxygen indicators. Purging the instrument with fresh air helps remove any residual gases or contaminants that may have accumulated during the previous measurement, preventing cross-contamination and ensuring the next reading is not affected by the previous sample. The other options are incorrect because: B) the tested compartment's air may still contain the same gases or contaminants that were just measured, defeating the purpose of purging; C) CO2 is not the appropriate purging gas for an oxygen indicator; and D) water would not be an effective or appropriate medium for purging the instrument.