Answer:
Legend.
Step-by-step explanation:
From the narration given, two young men were twins and one of them died, and as a result, the other twin couldn't live without the other one, so Zeus changed them both to twin stars.
This is an example of a legend because a legend is simply an exaggeration of a tale to give the characters a larger than life quality such as is impossible to be true.
The fact that Zeus is said to have turned them to twin stars makes it fantastic and thus, a legend.