In 1357, Chaucer became a page at the court of which king?
Edward III was the King of England in 1357. The Earl and Countess of Ulster were his guardians, and that's when he was page to the countess. This was so because a lot of Chaucers close relatives died during the Black Plague, leaving his Dad with family heirlooms, properties, and money. So, automatically Chaucer would have had rights as an upperclass man or upper-middleclass man to serve a person of the aristocracy, as it is was considered a "worthy" thing to do for at the time.
Does your homework actually say he was a page at the court of a king? Or does it just ask who he was a page for? Nothing I can find says he was ever a page for a king. Instead, the things I can find say that he was a page in 1357 (the first time he is ever mentioned in history) but not at the court of a king. Instead, he is listed as a page in the household of the Countess of Ulster. Her husband is listed as Prince Lionel, but not as a king.
The earliest known document in which Geoffrey Chaucer is named is a household account book kept for the Countess (June 1356 - April 1359). The account book mentions purchases for Geoffrey Chaucer in April 1357: a short jacket, a pair of red and black hose (stockings) and a pair of shoes.