Understanding 'Serendipitous' in Context

The word serendipitous often conveys a sense of happy accident or unexpected good fortune. In a recent class discussion, the teacher described a situation where a scientist happened upon a critical discovery while researching something else. This fortuitous event not only led to a major breakthrough in their field but also illuminated the importance of open-mindedness in research.

Given this context, which of the following best captures the meaning of serendipitous as used in the discussion?