Final answer:
Immanuel Kant defined enlightenment as "man's leaving of his self-caused immaturity". He emphasized the importance of reason and human experience in philosophical thought. Therefore, the correct option is a. Immanuel Kant
Step-by-step explanation:
The philosopher who defined enlightenment as "man's leaving of his self-caused immaturity" was Immanuel Kant. Kant was a key figure in the Enlightenment period, attempting to move beyond the failures of traditional philosophy and metaphysics by emphasizing the relationship between reason and human experience.
He believed that individuals should rely on their own reasoning capabilities rather than accepting authority or dogma.