[elbhy]

المشاركة الأصلية كتبت بواسطة H0riz0n

My pen friend, who lives in Sweden, is studying biology at university

Using non-defining relative clause means that the friend is known and this means that he has only one friend.
But if the sentence is written without commas like this:

My pen friend who lives in Sweden is studying biology at university
In this case the relative clause is defining and it means he has many pen friends, but here he is talking only about the Swedish one.

So the answer is: a) one pen friend

I completely agree