If someone is "nonplused", what are they experiencing?

When someone is described as "nonplused," they are experiencing bewilderment or confusion. The term implies that the individual is so surprised or puzzled that they don't know how to react or respond. This state can arise when unexpected or perplexing events occur, leading to a moment of being stunned or at a loss for words. The word effectively captures a sense of being caught off guard, where the person might be unable to process what is happening or make sense of the situation. Thus, it aligns closely with the notion of bewilderment, making it the correct choice in this context.