the human brain is an............. complicated organ (credit)

The correct word to fill in the blank is "incredibly."

The sentence would read: "the human brain is an incredibly complicated organ."

The choice of the word "incredibly" emphasizes the extraordinary complexity of the human brain. The brain is composed of approximately 86 billion neurons and trillions of synapses, which create an intricate network responsible for various functions such as thought, memory, and coordination. Using "incredibly" conveys the idea that the level of complexity exceeds what is typically encountered, highlighting how advanced and remarkable the brain is as an organ.

In summary, the word "incredibly" fits the sentence grammatically and contextually, effectively expressing the profound intricacies involved in the functioning of the human brain.