答案：D

题目分析：考查定语从句的变形： 介词+which引导的定语从句可以改成介词+which+不定式的结构做定语，

句意：在圣诞前夜，她只有2美元可以给丈夫买个简单的礼物。先行词是$ 2，用with which she could buy改成with which to buy做定语，选D。

点评：介词+which引导的定语从句可以改成介词+which+不定式的结构做定语，如：The beggar has no money that he can buy food with. ="The" beggar has no money to buy food with. ="The" beggar has no money with which to buy food，介词+which/whom +不定式”结构，在这种结构中介词只能放在which/whom的前面，不可以后置