Final answer:
The word 'impulse' is best defined as a sudden urge or inclination, which means it is an immediate and instinctive response to a situation, contrasting with premeditated or delayed actions.
Step-by-step explanation:
The correct definition of the word impulse in the context of 'The American's first impulse was to warn the others' is b) A sudden urge or inclination. An impulse is a spontaneous reaction that someone feels without a lot of premeditation or planning. It is that immediate and instinctive urge to do something. This contrasts with a predetermined action, a carefully planned decision, or a delayed response. Impulses can lead to immediate actions and are often based on instincts or emotions.
The effect of impulse becomes clear when we consider its use in physics, where it describes a force acting over time that changes an object's motion, linking the term with sudden and possibly powerful changes. Applying this concept metaphorically to human behavior, an impulse can suddenly propel a person to act.