Answer :
Sentence C
Sentence D does not need a comma after the word back
Sentence B does not need a comma after the word dog
Sentence A needs a comma after the word soup
Sentence D does not need a comma after the word back
Sentence B does not need a comma after the word dog
Sentence A needs a comma after the word soup
The sentence that is written correctly is the following one: The envelope, which had no return address, was yet sitting on the table at the foot of the staircase. The phrase which had not return address is not necessary or essential to the meaning of the sentence and therefore should be set apart by commas.