The B or * C * trains are closer - the D does not stop at 81st Street. You can also take train 1 to 79th Street and walk two blocks (between the museum on 81st Street or Central Park West). Enjoy your visit.
It all depends on where you are, you have to take line B or D of the metro and the stop is inside the museum on 81st Street, it is called 81st Museum of Natural History
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