Which Child Is Exhibiting Underextension

Which child is exhibiting underextension

A)  Britney, who uses the word "car" only when referring to her mother's car 
B)  Abby, who uses the word "swing" when referring to all the pieces of playground equipment on the playground 
C)  Danielle, who uses the word "puppy" when referring to all dogs, young or old. 
D)  Caitlin, who uses the word "doggie" when referring to both her dog Fido and her grandmother's cat Fluffy