The missing word in the sentence should be "Knowingly," so the complete sentence is:
"A person acts knowingly with respect to the nature of his conduct or to circumstances surrounding his conduct when he is aware of the nature of his conduct or that the circumstances exist. A person acts knowingly, or with knowledge, with respect to a result of his conduct when he is aware that his conduct is reasonably certain to cause the result."
So, the correct answer is Option 2: Knowingly.