Synaesthesia Does the Number Five Smell Like Lemons? Is the Sound

Synaesthesia
Does the number five smell like lemons? Is the sound of a flute purple? Does the letter "R" taste like cinnamon? These questions might seem strange, but to people with synaesthesia, they make perfect sense.
Synaesthesia is a neurological phenomenon where the senses get crossed. People with synaesthesia (also called "synaesthetes") experience this phenomenon in different ways. The most common form of synaesthesia is called color-graphemic synaesthesia. People who experience this form of synaesthesia associate letters and numbers with certain colors. For example, the letter "A" might always appear to be red and the number eight might always look blue. Another common form of synaesthesia is chromesthesia: the association of sounds with colors. Some people who experience this phenomenon see bursts of color when they hear everyday sounds, such as dogs barking or water dripping. Others associate specific colors with different musical notes.
Although scientists do not yet understand what causes some people to experience synaesthesia, some researchers argue for something called the cross-activation theory. This theory suggests that the regions of the brain responsible for interpreting information from the different senses have become "crossed," that is, the neurons from one sense inadvertently activate the neurons from another sense. There is also evidence to suggest that synaesthesia is a genetic trait. While there is no treatment for synaesthesia, there is also not much demand for a treatment; most synaesthetes report that they find their condition rather pleasant.
Question: What is the author's purpose in writing this passage?

A) To entertain readers with a funny story about synaesthesia.
B) To persuade readers of the potential hazards of a rare mental disorder.
C) To encourage readers to show empathy toward those with synaesthesia.
D) To define an unusual condition and explain its possible cause.