Thatwasthereason
其实你所说的两道题都要用that，当然which或省略也行。
That was the reason _____ Jane refused to speak for at the meeting。
象sweetmich所说的，要注意介词for，如果把句子稍微改一下，应该就很明了了
That was the reason for_____ Jane refused to speak at the meeting。 只是这样一来就只能用which
后来你又举了一题That was the reason ________ Mark refused to talk about at the...全部
其实你所说的两道题都要用that，当然which或省略也行。
That was the reason _____ Jane refused to speak for at the meeting。
象sweetmich所说的，要注意介词for，如果把句子稍微改一下，应该就很明了了
That was the reason for_____ Jane refused to speak at the meeting。
只是这样一来就只能用which
后来你又举了一题That was the reason ________ Mark refused to talk about at the meeting ?A。
why B that 你一样要注意到about后面缺了宾语，也就是说
the reason 在定语从句中充当的是宾语成分（那就是Mark不愿意在会上谈及的原因，绝对不能翻成那就是为什么Mark不愿意在会上谈及的原因）再来看一下此题的变化That was the reason ________ Mark refused to talk about it at the meeting ?A。
why B that 这样一来就只能用why了
从你的问题中可以感受到你对句子的成分不是很理解。真正要学好定语从句，成份的把握是很重要的，which，that等是关系代词，做主语或宾语；why，where，when是关系副词，做状语。
“reason后的定语从句的关系词是why一点没错”这句话也要加一个字“reason后的定语从句的关系副词是why（=for which）一点没错”
至于wangbeiqi456所说的“当先行词为:the reason是可以用that代替why”当然是对的，但和你这题没关系。
因为他说的那种情况为避免重复才会出现的一种特殊用法。收起
