Answer:
Odyssey
Step-by-step explanation:
First you want to find the meaning of each word and determine what will BEST fit into the sentence
Odyssey - A long wander or voyage usually marked by many changes of fortune.
Vain - having or showing an excessively high opinion of one's appearance, abilities, or worth.
Siren - a device that makes a loud prolonged sound as a signal or warning.
Herculean - requiring great strength or effort.
We can eliminate vain and siren
we are now left with the 2 likely answers, herculean and odyssey.
"The month-long trip "
knowing that statement above and "A long wander" from the definition of odyssey , we can conclude that the answer is odyssey!