Final answer:
The correct sentence is 'Shelly, who had so much work to do, decided not to attend her friend's birthday party.'
Step-by-step explanation:
The sentence that shows the correct use of a comma in a compound sentence is:
Shelly, who had so much work to do, decided not to attend her friend's birthday party.
In this sentence, the commas are correctly used to set off the non-essential clause 'who had so much work to do'. The commas indicate to the reader that this information can be removed from the sentence without changing the main meaning.
Learn more about Correct use of commas in compound sentences