The principle excuses harm caused to a person if the act was done in good faith and for that person's benefit, even without their consent. B, a passerby who had never used a gun before, picked up A's gun in good faith while A was being dragged by a lion, and the shot meant to save A ended up injuring A instead.
Since B's act was undertaken in good faith to benefit A during an emergency, the principle shields B from liability regardless of the lack of consent or B's inexperience with firearms.
Therefore, the correct answer is B is not liable as he has done the act in good faith.
The fact that controls this question is B's state of mind and purpose, B picked up A's gun in good faith specifically to save A from the lion. Neither B's skill with firearms nor A's consent appears anywhere in the principle's requirements, so each option should be tested against good faith and benefit alone.
Running each option through the good faith and benefit test, rather than through skill or consent, leaves only the option that shields B from liability.
Therefore, the correct answer is B is not liable as he has done the act in good faith.