Good bus and answers. Trains/subways...B, Q, D, N, R, 4, 5, LIRR trains to Atlantic Avenue/Barclays Center to transfer to 2, 3 trains to Grand Army Plaza. B, Q, S to Prospect Park. F, G to 15th Street/ Prospect Park. These stops from Manhattan to Brooklyn (south) routes. Also Brooklyn to Manhattan (north) routes.
Bus 41 will take you Prospect Park. The 3 and the 2 train will also take you there. Then there's the R train.
You can take the train to grand army plaza
How can u get there
Adding to Ray Guns answer, it also depends on where you are coming from. Speaking very broadly, the B41 runs along Flatbush Avenue, while the B68 comes up from the south, along Coney Island Avenue. There are many others as well.
That depends on where in the park you're trying to go. It's a large enough park that there are different bus/subway lines appropriate for different areas
Thanks! Your answer is awaiting moderation.