Answer:
1. "Cool things that computers can't do"
AI is Artificial Intelligence, which can involve anything from robots to programs on computers/websites. Sure, AI can learn things and adapt in ways unlike computers, but they are still quite similar in the fact that they both are technology.
2. "Machines imitating intelligent human behavior"
This is a bit closer to the definition of AI. However, AI does not necessarily have to imitate humans - rather, it's most known for being able to adapt and learn (which is the part where this definition is correct). Also, our current AI is nowhere near the level of mimicking intelligent human behavior enough to fool us.
3. "Autonomous and adaptive systems"
This is, by far, the best definition given here yet. Most AI are autonomous and/or adaptive systems, used to automate tasks which would take time for us humans. I suppose one thing that could be wrong with it is that not all AI are made to be autonomous systems.