Final answer:
Archaeologists are left to 'speculate about' what happened regarding the complete set of pessoi found in modern-day Israel, making option C the correct choice.
Step-by-step explanation:
In 2008, a set of ancient pessoi (glass game pieces) was found in modern-day Israel, intact beneath a paving stone. Since pessoi were easily lost due to their small size, finding a complete set is unusual and has led experts to think that this set may have been intentionally buried. However, without definitive evidence to confirm why they were buried, archaeologists are left to speculate about what happened. So, option C, 'speculate about,' completes the text with the most logical and precise word or phrase.