The principle protects a person who causes harm to someone else, as long as two conditions are met together, good faith and intent to benefit that person, and it specifically says consent is not needed for this protection to apply.
B saw A being dragged by a lion and picked up A's own gun to fire at the animal, purely to save A. That act was done in good faith and squarely for A's benefit, which satisfies both conditions the principle sets, and the fact that A never gave consent to being shot at, or to B using the gun, does not matter because the principle already accounts for that.
B's lack of prior experience with a gun does not change any of this, since skill level is not one of the conditions the principle tests. What matters is the motive and good faith behind the act, both of which point toward B, so none of the liability-based options survive contact with the actual wording of the rule.
So the correct answer is B is not liable as he has done the act in good faith.
Checking what fact would need to be different for each option to hold shows where the incorrect choices go wrong.
Since good faith and benefit to A are already present and nothing about skill or consent changes that, the option resting on good faith is the one left standing.
Therefore, the correct answer is B is not liable as he has done the act in good faith.