They still rememered the years ( ) togeher in the country

如果D把for去掉 就可以了
不能‘for together’
如果硬要用'they had for'的话 就是
they still remembered the years the had for being together in the country.

你这个是定语重句吧，首先句子C选项少了一个they，正确的句子应该是
They still remembered the years (that) they spent together in the country,
D选项，for这个介词要去掉，had是及物动词，不加介词，改为they had就对了。