D seems best.
The utility company has announced that it will permanently close its Unit I nuclear power plant, the first plant that had been built by private industry and the model for a generation of modern nuclear reactors.
(A) that had been built by private industry and
notice the use of "that".
the first plant that X and that
Y ...... is the correct usage. you want to link two phrases to the same subject and you need to use relative pronoun that" in the second phrase .here thats not the case. A is gone
(B) built by private industry and which was
relative pronoun "which" is commonly used for a non-restrictive clause such as :
The pen , which is on the table, is a gift. here you can remove the clause starting with "which" and still the sentence wont be affect. thats not the case here.
Also, if we really wanted to use "which" here, we would need another "which" after "the first plant".... the same logic that could make A a correct choice.
Also, use of "was" may suggest that the plant is no longer a model for nuclear reactors
(C) to be built by private industry and which was
same thing with B. "to be" may be redundant.
(D) built by private industry and
Seems good to me.
(E) to have been built by private industry and was
use of "have been" is incorrect. "have been" is present perfect continuous tense. it suggests that an action started in the past and is STILL continuing.
this means, private industry started building the plant in the past and is still building it.
Please provide reasons for selecting an answer choice and rejecing other answer choices.
let me know if you need more explanations.