Final answer:
A semicolon should not be used to connect main clauses and replace existing conjunctions.
Step-by-step explanation:
A semicolon should not be used in option C, which states to connect the two main clauses of a sentence and replace the existing conjunction. In this case, a semicolon would not be appropriate because it is used to join closely related independent clauses, not to replace conjunctions. Conjunctions like 'and', 'but', or 'or' are used to connect independent clauses.
Learn more about Semicolon usage