Forest conservation is the practice of protecting forests and their natural resources for the benefit of the present and future generations. It involves a range of activities aimed at ensuring the sustainable use of forest resources while maintaining their ecological integrity. The options presented are examples of activities that can be carried out in forest conservation.
Prevention of bushfires, replacing harvested trees by planting seedlings, and selective exploitation of forest trees are all activities that contribute to forest conservation. However, extensive felling of forest trees is not considered a practice that promotes forest conservation, as it involves the removal of a large number of trees from an area. This can have negative impacts on the environment, such as soil erosion, habitat loss, and reduced biodiversity. Therefore, the answer is "extensive felling of forest trees."