Who _______(have) a pen?

答案：has.
解释：
1. 如果单独用who/what做特殊疑问句的主语,则其一般视为第三人称单数,所以此题谓语动词也用三单形式即has.
2. 给楼主举两个例子好了：
1) We are on duty today. I am on duty today.
如果对We/I划线提问,则答案为：Who is on duty today? 而不是Who am/are on duty today?
2）Many books are on the desk.
如果对many books划线提问,答案为：What is on the desk? 而不是What are on the desk?