Final answer:
According to EPA protocols, radon measurement device should be placed 20 inches away from an exterior wall.
Step-by-step explanation:
According to EPA protocols, the minimum distance a radon measurement device should be placed from an exterior wall is 20 inches. This is to ensure that the measurement device is not affected by the wall or any potential sources of radon in the wall. By placing the device at a sufficient distance from the wall, accurate readings can be obtained to assess the level of radon in the indoor air.