Explanation:

- Sentence Context:

The encounter on the bus was unexpected but had a significant impact.

- Underlined Word:

"Serendipitous" refers to something beneficial or fortunate that occurs by chance.

- Option 1: Fortunate

This captures part of the meaning, emphasizing the positive nature of the encounter.

- Option 2: Of historical significance

This suggests importance over time, not the element of chance or fortune.

- Option 3: Sudden and abrupt

This implies an unexpected event but lacks the positive or beneficial aspect inherent in "serendipitous."

- Option 4: A chance event

This accurately captures "serendipitous," emphasizing both the chance and potentially fortunate nature of the event.