http://en.wikipedia.org/wiki/Isabella_I_of_CastileIsabella I (Spanish: Isabel I, Old Spanish: Ysabel I; 22 April 1451 – 26 November 1504), also known as Isabella the Catholic, was queen of Castile and León (Crown of Castile). She and her husband, Ferdinand II of Aragon, brought stability to the kingdoms that became the basis for the political unification of Spain under their grandson, Holy Roman Emperor Charles V. . . . . Isabella and Ferdinand are known . . . . and for supporting and financing Christopher Columbus' 1492 voyage . . . .
Could this be a trick question? In the 14th Century there was no such place as "Spain". You had Castile, Aragon, Leon, Navarre, etc. Spain as a political unit didn't really exist until the late 15th century with the fall of Granada and the end of the Reconquista.
May 15th cent Spain, or rather what became Spain, would be a better context.
So it's perhaps best to consider what how the name was pronounced in Castilla y León.
I'm pretty sure Spanish-speaking people still call him Cristobal Colón, except when they're talking to English speakers in English.