定语从句关联词的选择主要取决于以下几个因素：

1. 先行词与关系词的指代关系：

如果先行词是人，常用关系词who, whom, whose。

如果先行词是物，常用关系词which, that, whose。

2. 先行词是否有限定作用：

如果先行词有限定作用，即先行词被特指，常用关系词that。

如果先行词没有限定作用，即先行词被泛指，常用关系词which。

3. 先行词是否是所有格：

如果先行词是所有格，常用关系词whose。

4. 关系词在从句中的语法功能：

关系词在从句中作主语、宾语或表语。

如果关系词在从句中作主语，常用关系词who, that。

如果关系词在从句中作宾语，可以省略，也可以用whom, that。

如果关系词在从句中作表语，常用关系词that。

以下是一些常见的定语从句关联词及其用法：

that：用于先行词被特指的情况，或关系词在从句中作主语或宾语时。

The car that you bought is very expensive.

This is the book that I have been looking for.

which：用于先行词被泛指的情况，或关系词在从句中作主语或宾语时。

The books which you gave me are very interesting.

I don't like the color of this dress, which is too bright.

who：用于指人，关系词在从句中作主语或宾语时。

The man who is standing there is my teacher.

The girl who won the prize is very intelligent.

whom：用于指人，关系词在从句中作宾语时。

The person whom you met yesterday is my friend.

I don't know the girl whom he is talking to.

whose：用于指人或物，关系词在从句中作定语。

The girl whose mother is a teacher is very clever.

The book whose cover is red is mine.

在选择关联词时，要根据具体情况综合考虑以上因素。