OFFICIAL EXPLANATION
The concert was a huge success, and towards the end of the evening the audience were at one with the band.A. and towards the end of the evening the audience were
Incorrect.
This answer choice is grammatically incorrect. The singular subject audience does not agree with the plural verb were.
What helps us identify this question as a Subject Verb Agreement question is the following Stop Sign:
A pseudo-plural singular subject
Audience is a confusing word. It refers to more than one person, which makes us think of it a plural subject, but it is, in fact, a singular noun (the plural is audiences).B. but towards the end of the evening the audience were
This answer choice repeats the original mistake. The singular subject audience does not agree with the plural verb were.
In addition, replacing and with but changes the meaning of the original sentence.C. but towards the end of the evening the audience was
While this answer choice corrects the original mistake and is grammatically correct, it changes the meaning of the original sentence by replacing the addition word and with the opposition word but. This change also produces an illogical sentence as the two parts of the sentence are not opposed to each other.D. towards the end of the evening the audience were
Incorrect.
This answer choice repeats the original mistake. The singular subject audience does not agree with the plural verb were.
In addition, this answer choice doesn't fit into the original sentence as a result of omitting the conjunction and.E. and towards the end of the evening the audience was
This answer choice correctly matches the singular verb was with the singular subject audience.