There are two species of sparrows that appear very similar to each other. a scientist tried tomate them. he observed that in spite of being of the same species, they didn’t interbreed. what evolutionary force must have worked in this example?
a) migration
b) geographical isolation
c) reproductive isolation

So, let's work through this one. The scientist is trying to mate two birds together. This means that he's probably just putting them both in the same cage. Since they're not going anywhere, it can't be migration. Likewise, they're not isolated from one another by location (unless it's a ridiculously huge, national-park-sized cage). By elimination, it's C. 

You also have to look at C and see if it makes sense, though. Reproductive isolation is when behavior or differences in anatomy prevent two individuals from mating, even though they would be genetically close enough to reproduce. The two birds are known to be the same species, which means they are very close, genetically. They are in the same cage, though, and they aren't breeding, which means something else must be preventing them. The best way to explain that would be that they are reproductively isolated.