Answer:
The answer is indeed the second option: Customers did not enjoy the service they received at the new restaurant.
Step-by-step explanation:
The reason why we may consider the second option to be formal and the others to be informal is the use of first-person and third-person subjects.
Formal sentences tend not to express the author's or speaker's subjective points of view. Thus, they tend to use third-person subjects.
Informal sentences, on the other hand, do not have such a restriction. They may express the speaker's personal opinion and, for that reason, they may use first-person subjects.
A.That new restaurant is the worst, and I'll never go back there again. --> first person.
B.Customers did not enjoy the service they received at the new restaurant. --> third person.
C.I didn't like the food at the restaurant and thought the service was really bad. --> first person.
D.The service at that restaurant is awful, and you shouldn't go there. --> the speaker, based on his/her own personal opinion, is telling the other person not to do something.