What synonym could best replace "suddenly" in this context?

The term "suddenly" refers to something happening quickly and unexpectedly. In this context, "instantly" captures the essence of an immediate occurrence, aligning well with the idea of something happening without warning or preparation. This choice emphasizes the promptness and lack of anticipation associated with the event described.

The other options do not convey the same sense of immediacy. "Gradually" implies a slow and progressive change, which contrasts with the abruptness of "suddenly." Similarly, "carefully" emphasizes caution and deliberation, which is inconsistent with the notion of something happening unexpectedly. "Slowly" also denotes a lack of speed, further distancing it from the meaning of "suddenly." Therefore, "instantly" is the most fitting synonym in this scenario.