The answer is:
At age nine, students began learning Latin, which was the language of international affairs.
Relative clauses provide additional information about something that has already been mentioned. In this case, the subordinate relative clause "which was the language of international affairs," makes a further comment about the previously mentioned Latin language.
It seems that you have missed the necessary options for us to complete this analogy, but hope this one helps. So if thread is to string, therefore, a cask is to barrel. A cask is a <span>large barrel like container made of wood, metal, or plastic, used for storing liquids especially those with alcohol. Hope this answers the question.</span>
It would make more sense if you did “later, she added a suggestion box.”
A - <span>he man who delivers our mail was late today. </span>
