Which weather condition is best described if it is neither hot nor cold, but features falling crystals?

The description given in the question matches the phenomenon of snow, which involves falling crystals. Snow forms when water vapor in the atmosphere freezes into ice crystals, which then cluster together and fall to the ground. This occurs in conditions that are typically cold but can also be described as having a neutral temperature, where it doesn't feel distinctly hot or cold to some people.

Snow is characterized by its unique crystalline structure, which causes it to appear as delicate, beautiful flakes when it falls. This aspect of falling crystals directly aligns with the answer. The other weather conditions mentioned do not describe falling crystals: rain consists of liquid droplets, windy conditions do not inherently include any precipitation, and sunny weather generally indicates clear skies without any form of precipitation. Therefore, the correct choice highlights the specific feature of snow that distinguishes it from other weather phenomena.