A.
People in the city ride the bus.
People is third person plural which takes the third/plural form of the verb ride, which is also ride. This sentence has subject/verb agreement.
B.
A box of old photographs were found in the attic.
A box is the singular subject. The dependent clause, "of old photographs" describes the box (or what's in it). 'Were' is plural and needs to be changed to 'was' to agree with the subject.
C.
A quartet of young musicians have won several awards.
A quartet is one group (as evidenced by the singular article, 'A'. Therefore "have' needs to be changed to 'has' for s/v agreement.
*NB- In British English this sentence would be acceptable. I haven't figured out why, just yet-- possibly because rather than a group being considered as one unit they see it as all the individuals in the group.
D.
Our new dog from the shelter were sneaky.
The subject of this sentence, dog, is singular. Therefore the verb, 'be', must also be singular. "Our new dog from the shelter was sneaky.
