Here's why:
* Unanticipated implies that the meeting was unplanned and unexpected. Elizabeth and the Gardiners were visiting Pemberley out of curiosity and to see the estate, not knowing that Darcy would be there.
* Awkward might be a good choice, but it doesn't capture the element of surprise.
* Pleasant is too general and doesn't fully convey the impact of the meeting.
* Unforgettable is a strong word that suggests a dramatic encounter, but it doesn't highlight the element of surprise.
The meeting at Pemberley was a pivotal moment in the novel, as it allowed Elizabeth to see Darcy in a new light and begin to change her opinion of him. The fact that it was unanticipated adds to the significance of the encounter.