Final answer:
King Arthur and his knights could have been doing a number of things when interrupted by an unexpected guest; most likely they were at a feast (option a) if this is about the Holy Grail narrative, as the provided context does not give specific details to align with the options.
Step-by-step explanation:
The student is asking about an event from Arthurian legends involving King Arthur and his knights. The context provided does not precisely match any specific story from Arthurian lore, as these legends have been told in many variations. However, the options provided suggest that the student is referring to a narrative where King Arthur and his knights are interrupted by an unexpected guest.
Given the available options, the closest match, albeit not cited in the resources provided, would be the beginnings of the story of the Holy Grail in Arthurian legend, where the knights are gathered for a feast during the Pentecost at Camelot when they receive an unexpected guest leading to the quest for the Grail. This would correspond to a feast or celebration such as a wedding (option a) or they could simply be dining without any particular celebration. Without specifics, options b, c, or d are not supported by the provided information.