Which word describes something that is unlikely to cause harm?

The word that best describes something that is unlikely to cause harm is "innocuous." This term refers to actions, substances, or behaviors that are harmless or not likely to provoke any adverse effects. When something is classified as innocuous, it conveys a sense of safety and a lack of potential danger, making it a fitting choice for this definition.

"Benign" is also related to being gentle or harmless, particularly in a medical context, but "innocuous" more directly emphasizes a lack of harmfulness in a broader sense. The terms "malicious" and "hostile," however, suggest harmful intentions or aggressiveness, which directly contrasts with the idea of not causing harm. Thus, "innocuous" is the most appropriate choice given the context of the question.