Question

The woman to whom they __(talk)__(be)her mother.

Answer 1

The woman to whom they _talked_(talk)_is_(be)her mother. 他们与其谈过话的女人是她妈妈。

或
The woman to whom they _are talking_(talk)_is_(be)her mother. 他们正在与其谈话的女人是她妈妈。

或
The woman to whom they _have talked_(talk)_is_(be)her mother. 他们已经与其谈话的女人是她妈妈。

Answer 2

这是一个主语 带有定语从句 的句子。定语从句的引导词是whom 引导词在从句中做动词短语talk to 和……交谈 中to 的宾语，to 是介词，放在了定语从句引导词之前。
talk to的时态，可以是进行时态，也可以是过去时态，或者完成时态。