Question

Which pen do you want?The one on the table?(这句话不缺Is吗?）

Answer 1

这是一个省略句：

Which pen do you want?(Do you want )the one on the table?

翻译：你想要哪一支笔?你想要桌上的那支吗?

或者也可以表达为：

Which pen do you want?(Is it ）the one on the table?

翻译：你想要哪一支笔?是桌上的那支吗?

the one on the table中,on the table 是介词短语作定语,修饰"the one",译为“桌上的那支（笔）”