Which of the following is a potential sign of a radiological attack?

Materials that emit heat without any sign of a heat source can indicate a radiological attack due to the presence of radioactive materials. Radioactive substances can generate heat as they decay, which may not be immediately visible or associated with a traditional heat source. This characteristic can serve as a warning sign, as the heat produced can sometimes be detected by specialized equipment or even felt by persons nearby.

The other options, while potentially concerning in various contexts, are less directly indicative of a radiological threat. For example, glowing clouds or fog may be attributed to other atmospheric conditions or phenomena, and mutated plants or animals often signify environmental stress or contamination from a wide range of causes, not just radiological. Lastly, unusual drops of liquid on surfaces could originate from various non-radiological sources, making it a less specific indicator. Understanding these distinctions helps in identifying genuine threats related to radiological attacks.