A label for slightly toxic pesticides must carry which of the following words?

The correct answer is "Caution" because it is the designated label for slightly toxic pesticides under the guidelines of federal regulations. Pesticides are categorized based on their toxicity, and "Caution" is used for those that pose mild risks to humans and the environment. The use of this term indicates that while the product can be harmful, it is not as dangerous as those labeled with "Warning" or "Danger," which denote higher levels of toxicity.

In the context of pesticide classification, "Warning" is typically associated with moderately toxic products, while "Danger" is reserved for highly toxic substances that require more stringent safety measures. "Alert" is not recognized as an official category in relation to toxicity levels for pesticides. As such, "Caution" is the appropriate term to indicate that the pesticide is slightly toxic and requires careful handling by users.