Which word best describes a spontaneous act undertaken impulsively?

The term that best describes a spontaneous act undertaken impulsively is "impromptu." This word refers to actions or performances that are done without preparation or planning, embodying the essence of spontaneity and impulsivity. When something is labeled as impromptu, it indicates a decision made quickly and without prior thought, perfectly aligning with the essence of an impulsive act.

In contrast, other options represent the opposite of spontaneity. "Methodical" suggests a careful, systematic approach, while "planned" indicates that actions have been thought out in advance. Similarly, "calculated" implies a deliberate consideration of consequences and strategies before acting. These words convey a sense of control and forethought, which does not fit the concept of spontaneity clearly embedded in "impromptu."