The boat is moving at \(15\ km/h\) in still water and the current is moving at \(3\ km/h\)
Therefore, the boat's effective speed upstream = \(15\ km/h - 3\ km/h \) = \(12\ km/h\)
Now, Time taken by boat to go 36 km upstream = \(\frac {36\ km}{12\ km/h}\) = 3 hr
So, the correct option is (B): 3 hr.