Highly restricted interests are defined as:

Highly restricted interests refer to specific areas of fixation that an individual may have, particularly noted in various developmental conditions like autism spectrum disorder. These interests typically involve an abnormal intensity or focus, meaning that the person may engage with or think about these interests obsessively and may show little interest in other activities or subjects. This characteristic can significantly impact social interaction and daily functioning, as the individual may prioritize these fixated interests above all else.

In contrast, flexible interests that adapt over time do not qualify as restricted because they can change or evolve based on the person's experiences and environment. Similarly, interests that are diverse and varied imply a breadth of focus across different areas, which again contradicts the definition of being restricted or fixated. General interests in typical childhood activities suggest a healthy variety of engagement with a mix of activities, which is also not indicative of the fixation seen in highly restricted interests. Therefore, the definition capturing an intense and focused nature of interest is what makes the correct answer stand out.