Who was the king of Spain in 1540?

Spain did not exist as a single entity in 1450.

What is today Spain, more or less, was divided in 4 kingdoms, whose kings were the following:

- Castile, the biggest, covered most of the center and western part of Spain. It was ruled by King John II

- Aragon, the second in size, covered the eastern part. It was ruled by King Alfonso V.

- Navarre was a tiny kngdom in the western part of the Pyrenees. Its monarch in 1450 was John II (not the same John II of Castile), a son of the king of Aragon.

- Granada (though in paper it was an emirate, not a kingdom) was the last Muslim ruled state in the Iberian Peninsula. It covered what is today the south-eastern part of Andalusia. Its king (emir) was Muhammad IX. Granada paid tribute to Castile at this time and was more or less a vassal state of her.