Will go with A,
IMO A, trial is singular hence we need a singular verb "was"

Edited.
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IMO A ,

in B . use of were is wrong for singular subject.

In C , usage of past progressive is wrong

In D. again the use of were will change the meaning ..

in E use of were is wrong for singular subject trial

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The answer should be A.

Two key distinctions:
(1) Singular verb 'was' should be used as 'the trial' is singular
(2) Tense should be past perfect as the sentence relates two events occurring in the past together. In these events, the one that is clearly the earlier, will be in past perfect and the one that is later in simple past. The earlier tense is 'scientists had realized that the trial of the new polymer was to be postponed' and thus, this tense should be in past perfect. The later tense should be in simple past, which it is in the main sentence above (i.e., hundreds of samples continued to be sent to laboratories..)

Official Explanation:

This sentence is correct as written.

The underlined portion of the sentence contains the past perfect tense verb had realized, so check that the tense aligns with any time clues in the sentence. Because the non-underlined portion of the sentence contains the simple past continued to be sent, which occurs after the realization had been completed, the past perfect tense is correct. There appears to be no error in the original sentence, so keep choice A. Because there is no error in the original sentence, there are no obvious repeaters to look for, so evaluate the remaining answer choices individually, looking for reasons to eliminate each.

Choice B contains the plural verb were, which does not agree with the singular subject trial. This is a subject-verb agreement error. Eliminate choice B. Choice C contains the past progressive tense verb were realizing, which does not convey the difference between before and after. This is a verb tense error. Eliminate choice C. Choice D contains a partial form of the subjunctive mood were to realize, which incorrectly indicates a hypothetical or conditional situation. This is a verb tense error. Eliminate choice D. Choice E contains the plural verb were, which does not agree with the singular subject trial. This is a subject-verb agreement error. Eliminate choice E.

Choice A: Correct.

Choice B: No. The plural verb were does not agree with the singular subject trial. Subject-Verb agreement.

Choice C: No. The past progressive tense verb were realizing does not convey the difference between before and after. Verb tense.

Choice D: No. The subjunctive mood were to realize incorrectly indicates a hypothetical or conditional situation. Verb tense.

Choice E: No. The plural verb were does not agree with the singular subject trial. Subject-Verb agreement.

The correct answer is choice A.
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