Respuesta :
I think it would be "At age nine, students began learning Latin, which was the language of international affairs."
The best answer is the third option:
At age nine, students began learning Latin, which was the language of international affairs.
This connects the sentence in a fluid way, and also helps to establish the reason the students began to learn Latin, which is because it was the language of international affairs.
At age nine, students began learning Latin, which was the language of international affairs.
This connects the sentence in a fluid way, and also helps to establish the reason the students began to learn Latin, which is because it was the language of international affairs.