Here, we are required to state the prepositions in the sentence "Most of them had spent many hours working on their research prior to the due date".
The prepositions in the sentence "Most of them had spent many hours working on their research prior to the due date" are "of, on, and prior to".
Therefore, the prepositions in the sentence "Most of them had spent many hours working on their research prior to the due date" are "of, on, and prior to".
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