William has been working at H&S for the past 12 years. When some family trouble arose, William wanted to work in the afternoon shift for a month rather than his usual morning shift. The work performed in the afternoon shift was exactly the same as that in the morning shift and William had all the skills required to perform the job. One of his colleagues, Jack, who worked on the afternoon shift, was willing to swap shifts with William for a month but the supervisor did not allow William to change his shift. William reported the grievance to the union and it could not be resolved even after a long discussion with the management. Which of the following steps is most likely to be taken by the union as the final step in the grievance handling procedure?

a. Announce a strike.

b. Report the matter to the NLRB.

c. File a complaint against the employer in the court.

d. Request an arbitrator to resolve the grievance.

Please provide a reference and explanation for the question.