Which characteristic of trustworthy AI ensures that it functions reliably and consistently over time?

The characteristic of trustworthy AI that ensures it functions reliably and consistently over time is indeed characterized by being "valid and reliable." Validity refers to the extent to which an AI system measures or performs what it is supposed to, while reliability indicates that the system produces stable and consistent results across different conditions and over time.

These two elements are crucial in establishing trust in AI systems, as stakeholders need assurance that the outcomes from AI applications are not only accurate but also reproducible in similar contexts. For example, a healthcare AI tool must reliably provide similar diagnoses when presented with similar data, ensuring that it can be depended upon in clinical settings.

While the other options highlight important aspects of trustworthy AI, they do not directly address the consistent performance over time in the same manner. Safety relates more to the absence of harm, accountability and transparency focus on the governance and ethical dimensions, and explainability pertains to how well the workings of the AI can be understood. Hence, validity and reliability specifically emphasize the dependable operation of AI systems, making it the appropriate choice in this context.