Final answer:
Expressing empathy towards a friend involves primarily the affective domain, which relates to emotions and feelings, unlike the other options which are linked to cognitive and psychomotor domains.
Step-by-step explanation:
The question asks, "Which of the following describes a behavioral event that involves primarily the affective domain?" The affective domain is concerned with emotions and feelings. Among the options provided, expressing empathy towards a friend is the behavior that most involves the affective domain as it requires an understanding and sharing of the emotional states of others, an essentially emotional process.
The affective domain refers to the area of learning that deals with attitudes, values, and emotions. It is one of the three domains described in Bloom's taxonomy of educational objectives, the other two being the cognitive and psychomotor domains.
An activity such as solving a math problem would fall mainly under the cognitive domain as it involves logical reasoning and problem-solving skills. Memorizing a historical timeline is also a cognitive task as it involves recalling facts and information. Observing a chemical reaction is more about cognitive and psychomotor domains where observation and understanding of scientific processes are involved.
Therefore, when discussing activities that primarily engage the affective domain, we are talking about actions that pertain to feelings, emotions, attitudes, and values. In the context of the provided options, expressing empathy clearly represents an event linked with the affective domain.