In my view, there are two possible answers to this question. One has to do with European history while the other is more connected to the history of the Americas. You can choose which you think is the most important. Your choice should probably depend on what your class has studied about this time period.
One major event was the execution of Sir Thomas More (know called St. Thomas More by the Roman Catholic Church) in England. More was executed on the orders of King Henry VIII. He was executed because he could not agree with Henry’s attempts to take control of the Church in England. This is important because it was a big step in Henry’s move away from Catholicism.
The other major event was Jacques Cartier’s second voyage to the Americas, in which he went up the St. Lawrence River as far as what is now Montreal. This is important because it is a major event in the French exploration of what is now Canada.
One of these two is the most important event of 1535. You choose which one is more likely to be the correct answer for your class.