The word that best completes the given sentence is "VARIED" since the whole paragraph tells us that (...) "coral reefs are home to some of the most varied forms of life in the world". To reaffirm the sentence, it follows by saying that "thousands of different plants and animals have been found living among them".
The term extrinsic motivation originates in the character's outside environment. This type of motivation is a behavior driven by external factors to get any kind of reward or incentive. For example, some students study hard to pass an exam. They are motivated not for the sake of learning it, but because they want to pass an exam.
