Right answer:
b. ¿De parte de quién, por favor?
The context of this dialogue takes place when someone calls up to a place. Perhaps Mrs. Vargas lives or works here. In Spanish, when you call up to a place it is common to use the formula:
Name of the person you want to talk, + por favor
So if the person who picks up the receiver in the other side is not the person you need to talk, he would say the question:
¿De parte de quién, por favor? that translates into From who?, please?
