答案应该选C.
too...to...表示:太...而不能...后面加的是动词原形.
so...that...表示:太...以至于...后面加的是从句.
要区别他们的话只能说too...to...是在一个句子里的,而so...that...是在有从句的复合句里的.这个句子用too...to...也可以写成:
I am too busy getting ready for Christmas to write to you.
另外,这里还有几个词组:be busy doing sth.忙于做某事.
get ready for sth.为...做准备.