we use "X" as the ---for an unknown quantity.

In general, all three A,B, C can be used, but B 'symbol' is much more common.

The symbol "X" can also be called 'a mark' or 'a sign';

The following are all possible usages:

1. We use the symbol "X" to represent an unknown quantity. (more common)

2. We use the mark "X" to represent an unknown quantity.

3. We use the sign "X" to represent an unknown quantity.

4. We mark an "X" to represent an unknown quantity.

5. We sign an "X" to represent an unknown quantity.

6. We mark an "X" for an unknown quantity.

7. We sign an "X" for an unknown quantity.

以上7种方式都是可以接受的美语， 1 是最长使用的。

Hope this helps.

Cheers

Note: 本人已在海外生活了数十载。为了慎重起见，我的答案都与老美或老英交换过意见，你放心使用吧。如有任何问题，可补而述之。

这三个词都有标记和符号的意思;sign指能看到或感觉到的事物,也可指思想和身体状况;signal知警告,命令或给予信息的声音和动作,也可指引起某事发生的信号,