Based on the component code and requirements, the change that should be made to the Lightning web component is option B.
This option adds a new column for the MailingAddress field to the columns attribute in the javascript file, and also adds the MailingAddress field to the fields attribute in the HTML template file. This way, the Lightning web component can display the contacts' mailing addresses in the lightning-datatable component, as long as the Apex controller class returns the MailingAddress field in the query result. Option A would not work, as it only adds the MailingAddress field to the fields attribute in the HTML template file, but not to the columns attribute in the javascript file. Option C would not work, as it only adds the MailingAddress field to the columns attribute in the javascript file, but not to the fields attribute in the HTML template file. Option D would not work, as it adds the MailingAddress field to the wrong place in the columns attribute in the javascript file, and also adds an unnecessary comma after the last column definition. Reference: [lightning-datatable], [Call Apex Methods from Lightning Web Components]