Final answer:
The term 'belligerent' is best defined as to 'be aggressive', which fits the context of someone being quarrelsome and further stimulated by alcohol.
Step-by-step explanation:
The term belligerent relates to showing an inclination to be aggressive or to fight. In the context provided, where a person has been described as quarrelsome and whose natural combativeness has been awakened, it suggests that this person exemplifies belligerent behavior. This is especially true given the mention of the person drinking his fourth cocktail, which may imply increased aggressiveness due to alcohol consumption.
Therefore, concerning the multiple-choice question provided, the correct answer is c) to be aggressive. The other options given (assertive, irritant, and believing in something false) do not accurately define the term 'belligerent' as it is used in this context.