_______told you that was lying.

B、Whoever
谁告诉你这个，谁就在撒谎。

Whoever = anyone who
Anyone (who told you that = 定语从句) was lying.

A. 谁告诉你我当时在说谎的
题中that前应该漏了个I 或者其他主语

B、Whoever
无论谁和你这么说,都是在骗你.
Whoever (told you that) was lying.

C
任何人都告诉你,这是个谎言。

排除A因为他不是问句，两个主语怎么可能在一起~

B
陈述句，told you 后面加逗号，可省略