Concept:The correct choice must make it clear that only one of the two items can be possessed at the same time.
Explanation:The first part of the sentence gives permission: you may have the pencil.
The second part adds a restriction: you cannot have the ballpoint in addition.
"As well" means "too" or "also" and correctly expresses that the ballpoint cannot be added to the pencil.
"Either" does not fit here because it does not show the idea of having both items together.
"Furthermore" is only used to introduce an extra point, so it is unsuitable.
"Also" sounds unnatural when placed at the end of this sentence.
Therefore, the completed sentence is: "You may have the pencil, but you can't have the ballpoint as well."
Answer:A. As well