Linsey Hislop, 43, asked him about his health and how he was feeling as he continues to undergo cancer treatment.
He said: “I’m not too bad.”
The monarch was diagnosed with a type of cancer in February and has been undergoing treatment for it ever since.
The King took a short break and postponed his public-facing duties before returning to his normal schedule in April.
Meanwhile, an eight-year-old, Max, gave the King a small red silk heart he had made himself, prompting the monarch to gently pat his head in thanks.
Charles was also given some pink hearts crocheted by another well-wisher, June, who asked him to “give them to Kate and Charlotte”.
The King promised: “I will.”