What sign should be posted in a locked storage closet containing phantoms and sealed sources with a measured exposure rate of 3 mR/h?

The correct choice for posting in a locked storage closet containing phantoms and sealed sources with a measured exposure rate of 3 mR/h is "Caution: Radioactive Materials."

This is appropriate because the exposure rate of 3 mR/h is above background levels and indicates the presence of radioactive materials, even if they are contained within sealed sources. The labeling standards for radioactive materials require that areas where such materials are stored or used be clearly marked to inform individuals about the potential hazards associated with radiation exposure. This encourages proper safety protocols among personnel who may access the storage area.

While other options may reflect concerns about radiation exposure, they are not suitable for this specific rate. For example, "Caution: High Radiation Area" typically applies to areas where the radiation levels exceed 100 mR/h, which is much higher than the exposure rate mentioned. "Grave Danger: Very High Radiation Area" is used for extremely high radiation levels that could lead to serious health risks, and "No posting required" does not reflect the need for awareness and safety measures surrounding radioactive materials.