D可以看成是:not as a temporary expedient but as a seemingly permanent accommodation to the thing that many obnservers thought was an economy made over productive by advances in technology .
accommodation to the thing that many observers thought was an economy made over productive by advances in technology
"the thing"就是名词,后面"that"从句修饰"the thing" . "many observers thought"可以忽略不看而不影响句意。即“the thing that was an economy ......"
"what many obnservers thought was an economy made over productive by advances in technology ." 其实就是"the thing that..."的变体,两者没什么区别。无非是省略了“the thing that" 因此你可以把后面一整句话看成一个noun phrase