Answer:
C: "Well, if you'd told me, " David mumbled, "we could have worked it out."
Step-by-step explanation:
If you're going to interject with who's saying the sentence in the middle of it, you always put a comma and quotation before saying so-and-so said, or whatever. And after you say who said the sentence, you always put a comma and quotation. And finally, once you finish the sentence, you always and with a period and quotation.