Sentence Correction

In a previous design, the weight of the discus used in track competition is concentrated in a metal center, but now it is lined with lead around the perimeter, thereby improving stability in flight and resulting in longer throws.

A.In a previous design, the weight of the discus used in track competition is concentrated in a metal center, but now it is
B.According to a previous design, the weight of the discus used in track competition was concentrated in a metal center, but now it is
C.Once designed with its weight concentrated in a metal center, the discus used in track competition is now
D.This discus used in track competition, once designed with its weight concentrated in a metal center, but now
E.The discus used in track competition was once designed having its weight concentrated in a metal center and now

Answer is C.

Why is D wrong?

Best answer appears to be "C." Answer A is the wrong tense when referring to a previous design, B is technically gramatically correct but is awkward since the first part of the sentence refers to a design and the second to the discus itself, and D is a gramatical nightmare, never connecting a verb to the original subject. E needs the "it is" to remain intact to sustain grammatic correctness in the second half.

'This' is a pronoun, but we don't have any preceding noun to refer in option D. Also, D is an incomplete sentence. "This discus..." doesn't have any verb part. Therefore, D is out.

The sentence requires two tenses-past tense to describe the previous design and present tense to describe the current design of the discus. In the sentence presented here, the grammatical subject is weight, not discus, so the pronoun it
grammatically refers to weight, which makes no sense.

A The verb tense describing the previous design should be past, not present; the pronoun it refers to the subject of the first clause, weight, and it does not make sense to say that the weight of the discus is lined with lead.

B Although the tense of the verb in the first clause is appropriately past, the pronoun in the second clause is still referring to weight instead of discus.

C Correct. The introductory phrase describes the past design of the discus with a past participle (designed) that modifies the subject of the main clause (the discus); the main verb is lined is in present tense.

D This version of the sentence is a fragment because it has no main verb.

E The sentence is missing a main verb in the final phrase; was once designed must be followed by is now lined.