The principle excuses an offence committed under intoxication only where the person was incapable of understanding the nature of the act, and only where that intoxication happened without the person's knowledge or against their will. A persuaded B and C, both first time drinkers, to keep drinking, and while intoxicated, C pushed B with full force, seriously injuring him.
Because B and C drank with full knowledge of what they were doing, the involuntary intoxication exception never comes into play, leaving C liable for the act of pushing B.
Therefore, the correct answer is C is liable.
The principle has two requirements that must both be met before intoxication excuses anything, incapacity to understand the act, and intoxication that came about without the person's knowledge or against their will. B and C drank knowingly after persuasion, which already fails the second requirement, so that single fact can be used to test every option.
Testing every option against the involuntary-intoxication requirement shows it is missing throughout, which keeps the exception closed and liability resting on the person who did the pushing.
Therefore, the correct answer is C is liable.