The demonstrations in Pariswas marred by clashes between police officers and bottle-throwing gangs of youths.
(A) was marred by clashes between police officers -->The word "Demonstrations" is plural hence "were" should be used.
(B) was marred by a clash among police -->Same as A
(C) were marred by clashes among police officers -->We are comparing two groups 1) Officers 2) Youths. Hence, Between should be used.
(D) were marred by a clash between a police officer
...
(A) was marred by clashes between police officers -->The word "Demonstrations" is plural hence "were" should be used.
(B) was marred by a clash among police -->Same as A
(C) were marred by clashes among police officers -->We are comparing two groups 1) Officers 2) Youths. Hence, Between should be used.
(D) were marred by a clash between a police officer
...
