Which of the following is an example of a simple idea?

The concept of an apple as sweet represents a simple idea because it involves a direct and straightforward perception of a characteristic of an object—specifically, the flavor profile associated with apples. Simple ideas are generally those that arise from direct sensory experiences and can be easily understood without complex reasoning or abstraction.

In contrast, the other options involve more complex ideas. The color red in a sunset can be considered simple in isolation, but it often evokes broader associations and interpretations relating to light, atmosphere, and emotional responses. The idea of justice in society is inherently complex, incorporating various social, philosophical, and cultural dimensions. Finally, the understanding of mathematics is abstract and multifaceted, involving patterns, structures, and rules that require higher-level thinking and reasoning. Therefore, the sweetness of an apple is a clear and immediate sensory experience, qualifying it as a simple idea.