Final answer:
Illumination is best defined as making something lucid or clear by throwing light upon it, corresponding with the root 'lucere'. Hence, the correct answer is option (b).
Step-by-step explanation:
The root "lucere" means to make bright or light up. Considering this definition, the best definition of illumination is 'To make lucid or clear; to throw light upon' which corresponds to option (b) of your provided choices. Illumination in various contexts such as art, science, and literature, often refers to the act of lighting something, making it clear, or decorating it with light or color, much like how an illuminated manuscript is decorated with elaborate designs and bright colors, or how scientific techniques use light to enhance the visibility of microscope specimens.
To further exemplify the term 'Enlightenment' in a sentence: The sudden enlightenment he experienced after understanding the complex concept was as if a light bulb had lit up in his brain.