Read the sentence. the month-long trip across the desert was quite a(n) which word works best in this sentence? odyssey. vain. siren. herculean.

Odyssey

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!