When would a researcher be most likely to observe resource partitioning? between two allopatric populations that eat the same thing between two sympatric species that eat similar-sized seeds between two sympatric species, one herbivore and one carnivore between two species, one predator and one prey?

The answer is ‘two sympatric species that eat similar-sized seeds’. Sympatric speciations occur when a species population diverges without a physical barrier. Resources partitioning could occur when individuals in a population compete for same resources. Some individuals in that population may begin to utilize different resources to minimize the competition (and avoid being competitively excluded from the population).  Over time, this results in a divergence of the populations that utilize different resources hence carving out different niches.