Answer :
The third sentence. It sounds clumsy - like you are talking to a friend, just "chillin'".
Answer:
The statement which should be revised for a more formal tone is the following one:
Amaltheia was kind of like a goat who fed Zeus, which Zeus was really happy about.
Explanation:
It sounds way too informal, and there is a familiarity tone which should be avoided when one wants to sound formal.
"Kind of like" and "was really happy about" sound like coming from a chat between close friends, so they are inappropriate if a more formal tone is sought. Therefore, the aforementioned statement is the one which should be revised for a more formal tone.