In the context of AI, what is an expert system?

An expert system is defined as a computer program that uses artificial intelligence to solve complex problems by reasoning through bodies of knowledge, represented mainly as if-then rules. The key aspect of an expert system is its ability to simulate the decision-making ability of a human expert in a specific domain. This involves analyzing data and applying rules to draw conclusions or provide recommendations, which aligns closely with the idea of a system that combines analysis and activity.

The focus on analysis refers to the system's capability to assess information, evaluate conditions, and generate responses. The activity part emphasizes its application in real-world scenarios, where it assists users in making decisions or performing tasks that typically require human expertise. Thus, the answer highlights the integrated nature of reasoning and action within expert systems, making it the correct choice.

Other options do not fully capture the essence of what an expert system encompasses. For instance, systems geared solely towards physical tasks, human interaction, or observation lack the critical element of reasoning based on specialized knowledge, which is central to expert systems.