Answer:
Jillian
Step-by-step explanation:
The sentence says that Jillian smiled ear to ear while taking the adorable puppy.
Otherwise, we could use the process of elimination:
It couldn't be "Her" because the sentence is "her hands."
It couldn't be "him" because "him" is the dog.
It couldn't be Knuckles because
1. Jillian was the one smiling
2. a dog probably couldn't smile ear to ear (that would look creepy)
Hope this helps :)