OFFICIAL EXPLANATION
The original sentence contains a subject-verb agreement error. The singular verb "was" does not agree with the plural subject "demonstrations."
(A) Incorrect, as it repeats the original sentence.
(B) This choice contains the same subject-verb agreement error as in choice (A). The singular “was” does not agree with the plural "demonstrations." Also, “a clash” changes the original meaning, as the original "clashes" suggested more than one clash at more than one demonstration. Finally, “among” is incorrect, as it should be used in reference to three or more entities. “Between” should be used for two entities or sets of entities: "police officers" and "bottle-throwing gangs."
(C) This choice corrects the subject-verb error; the plural “were” agrees with the plural “demonstrations.” However, “among” is incorrect, as it should be used in reference to three or more entities. “Between” should be used for two entities.
(D) This choice corrects the subject-verb error; the plural “were” agrees with the plural “demonstrations.” However, “a clash” and “a police officer” both make the intended meaning less clear and somewhat absurd. The original meaning was that there were multiple clashes at the multiple demonstrations, and that they involved more than one police officer opposing “gangs” of youths.
(E) CORRECT. This corrects the subject-verb agreement error. The plural “were” agrees with the plural ‘demonstrations.” The choice contains no other errors.