the problem is analogous to the one below.
vijay,who is a graduate,who was preparing for GMAT,who joined the beatthegmat community.

this is not a complete sentence because the action that was done by the subject of the sentence has not been mentioned,

vijay,who was preparing for GMAT,who joined the beatthegmat community, is a graduate.

now this is a complete sentence, so in the former sentence a main verb is missing while in the latter it is present.

hope this helps...

Also, "which" is incorrect usage for this sentence.

of the remaining - only D makes sense

Why is "which" incorrect, it refers to the noun phrase: science of economics.

brick2009 wrote:Also, "which" is incorrect usage for this sentence.

of the remaining - only D makes sense

The science of economics, which for four decades was dominated by Keynesians, who at first stressed the government's role in stimulating the economy, but who were ultimately led away from solutions based on government intervention.

(A) economics, which for four decades was
(B) economics that was to be
(C) economics, one which has, for four decades, been
(D) economics is one that for four decades has been
(E) economics, for four decades, is one that was

This one is straightforward and simple,

Whenever there are two events occured in the past and one of the event has occured prior to other other event, you need to use past perfefect(had been) and both the event have occured in the past( getting dominated by Keynesians and who were ultimately led away from solutions....) but i dont see past perfect rather present perfect, so i choose D which has been.

Also, if i try POE approach, A,B,E are eliminated as simple past is enought since two past events mentioned. You need to use simple past tense for only one event like

my dad gave me money yesterday

vs

My dad had given me money before he realized that my mom also gave money to me.

hope this explains