Final answer:
The missing word to classify hair texture by diameter is 'medium.' The choices provided refer to the pattern of hair, not the diameter. The example of measuring hair diameter helps understand the small scale being discussed.
Step-by-step explanation:
Hair texture is measured by the diameter of the hair strand and is classified as coarse, medium, or fine. The option that best fits in the blank from the choices provided is not explicitly listed; choices such as wavy, curly, straight, and frizzy describe the shape or pattern of hair, but not its diameter. However, based on typical classifications of hair texture, the term 'medium' is the correct descriptor to use when discussing the diameter of hair strands between coarse and fine.
To put this into perspective, Example 2.3.1 helps us understand scale: a human hair has a diameter of about 6.0 × 10-5m. An appropriate unit of measurement could be micrometers (μm), and thus the diameter in micrometers would be 60 μm. This comparison highlights that even a tiny length of hair, having substantial mass, can provide an interesting reference point for measurements at a microscopic scale.
Therefore, the correct answer to fill in the blank would be 'medium', but it's important to note the original list did not contain this option.