D since 他告诉我他已经好几年没有那么高兴了.
如果没有he told me that
那么句子应是现在时it is years 或 it has been years since he enjoyed himself so much.这里since表示从过去某个时间点到现在这段时间里发生的事
加上了主句he told me that后面从句都变为过去时,所以是He told me that it was years since he had enjoyed himself so much