Final answer:
In MLA style, for works with more than two authors, you use 'et al.' after the first author's last name. The completed sentence would properly include 'Joel Best et al.' to reference the work by multiple authors.
Step-by-step explanation:
To complete the sentence referencing a work by Joel Best and others using MLA style guidelines, the correct option is 'a. et al.'. This is because 'et al.' is used for works with more than two authors, where only the first author's last name is provided followed by 'et al.' So the sentence would read: Joel Best et al. discuss the role that the issue of student loan debt played in the 2016 Democratic campaign (372).