Final answer:
The correct em dash usage is when it sets off an abrupt change or additional thought within a sentence. None of the given options use the em dash correctly. Commas should be used to separate independent clauses, after introductory elements, and in lists.
Step-by-step explanation:
The correct use of an em dash in the given options is none of the above. The em dash should be used without spaces on either side to set off a part of the sentence that is an abrupt change in thought or as an additional thought, not to replace missing words or to connect hyphenated words that aren't compound modifiers. An example of correct usage would be: 'He didn't hear the doorbell—since it was broken—when the delivery person arrived.' In this sentence, the em dashes are used correctly to add additional information within the sentence.
Now, regarding the review questions, here is how you correctly use commas within them:
I finally found my keys, and I got to work just in time. (Comma before the conjunction 'and' that connects two independent clauses.)
Mrs. Contreras threw out her old coffee table and cleaned the carpet. (No comma needed because there is no independent clause following 'and.')
Taking the elevator to the roof, we hoped we could see the skyline and the bay. (Comma after the introductory phrase and no comma before 'and' since it connects two objects of the sentence.)
Though Susan wasn't feeling well, she went to the store anyway and bought ice cream, pizza, and candy. (Comma after the dependent introductory clause and commas in the series.)
I let my neighbor borrow my phone because she said hers was tapped by the police. (No comma is needed as the clause 'because she said hers was tapped by the police' is essential to the meaning of the sentence.)