Reasoning means reasons or a simpler way is to ask yourself “why?”
B. A prediction of the consequences for the position
I'm assuming that means a counter-argument, which is important for persuasive and argumentative essays.
He didn't want to anger Poseidon. Poseidon is the god of the seas and Polyphemus' father, and if he killed Polyphemus, he would anger Poseidon and his journey through the sea would be perilous.