Final answer:
John Adams conversed with a king, notably King George III, during his diplomatic missions in Great Britain.
Step-by-step explanation:
When John Adams was in Great Britain, he spoke with a king. At that time, the United States did not have a president or vice president, and the political system in Britain was a monarchy. As the United States was engaging in diplomatic relations with Great Britain, Adams would have likely engaged with King George III, who was the reigning monarch during Adams's lifetime, especially given that Adams served as the first American minister to Great Britain before becoming Vice President and later President of the United States.