D. Flow is a concept in psychology that describes a state of complete immersion and focus in an activity. When individuals experience flow, they often lose track of time and feel a sense of enjoyment and fulfillment. This state is typically achieved when a person engages in a task that matches their skill level with the challenge presented. Flow can occur in various activities, such as sports, art, or music. It is characterized by clear goals, immediate feedback, and a sense of control. Achieving flow can enhance creativity and productivity, making it a valuable experience in both personal and professional settings.