Is this a prepositional phrase: the citizens and the newcomers met downtown in whoville.
if yes, where. if not, how can i turn it to a prepositional phrase?

"In Whoville" is a preositional phrase. It indicates where or when. A prepositional phrase will begin with a preposition and end with a noun.
In= preposition
Whoville= noun

D) She feels much happier when she sees her mom. 

The theme of the story is about how your family and friends are more important than your belongings. While Sydney's sister was worrying about saving her things, Sydney was worrying about the safety of their mother.