A boat is containing leaves. When they are thrown into the water, the level of water in the lake                                                                                                  

The boat containing leaves while floating in water would have displaced weight of water equal to (Boat + leaves) weight when leaves are thrown into water now they float on their own displacing water equal to their individual weights. We get the total volume of water displaced by total weight of (Boat + leaves) together is equal to volume of water displaced by boat and leaves separately. So the level of water lake remains unchanged.