Final answer:
No comma should be added to the sentence "Careers that require special training or a college degree often pay above-average salaries," as the clause is essential to the sentence's meaning and it does not list multiple items that would require a serial comma.
Step-by-step explanation:
In the sentence "Careers that require special training or a college degree often pay above-average salaries," no comma should be added. According to punctuation rules, it is important not to place commas around information that is essential to the meaning of a sentence. In this case, the clause "that require special training or a college degree" is essential to the meaning of the sentence as it defines the type of careers being talked about. Adding a comma could potentially alter the intended meaning or make the sentence less clear. Additionally, it is a common error to place a comma after the subject of a sentence, which should be avoided unless the subject is followed by a nonessential clause or introductory information.
Serial commas, or Oxford commas, should be used for clarity between items in a series, but this is not necessary in the provided sentence as there is no list. Moreover, when using a comma with coordinating conjunctions to join main clauses, such as in the case of conjunctions like 'and' or 'or', the rule applies only when they join two independent clauses, which is not the situation here.