He used to keep a notebook,in which----the namesof his frien

答案：B.
翻译：他过去常常保留一本笔记本,上面写了他朋友们的名字.
此题考查定语从句和倒装句型.
1.非限制性定语从句中的关系代词which代指先行词notebook,放入定语从句中为：
in which were written the names of his friedns
= the names of his friends were written in which
= the names of his friends were written in the notebook
2.定语从句采用了全部倒装,即把状语in which提前,中间是谓语were written,主语the names of his friends置后.
句子主语the names of his friends为复数,时态为一般过去时,所以be动词用were.