Final answer:
The preposition that best completes the sentence is 'sous' which means 'under' in English.
Step-by-step explanation:
The preposition in this sentence should be sous which means 'under' in English.
So, the correct sentence should be 'Il n'y a pas de monstre sous le lit.'
This preposition indicates that there is no monster under the bed.
Learn more about Prepositions in French