Which of the following represents a long-term testing method for measuring radon?

Long-term testing methods for measuring radon are designed to provide a more accurate assessment of radon levels over an extended period, typically at least 90 days. The reason for this duration is that radon levels can fluctuate due to various factors such as weather conditions and ventilation, and using a long-term test allows for the capture of these variations, resulting in a more reliable average radon concentration.

A 90-day test effectively smooths out short-term fluctuations and can provide a more stable average reading of radon levels in a home or building. This is crucial for understanding the overall radon exposure risk, as short-term tests, such as those conducted over 24 hours or 7 days, may not fully represent the year-round exposure that occupants may experience.

Immediate detection tests, while useful for quick assessments, do not give a true long-term picture of radon levels and are considered short-term tests as well. Therefore, the 90-day testing method aligns with the established guidelines for long-term radon measurement, making it the correct choice.