The correct answer is option 3, Eucalyptus. Eucalyptus is the tree species most commonly used in social forestry. However, it is important to note that the choice of tree species for social forestry may vary depending on the specific region and its environmental conditions. Eucalyptus is favored in social forestry due to its fast growth, ability to provide shade, and its potential for economic benefits. It is often planted in areas where reforestation is needed or for commercial purposes such as timber production. While Peepal, Gulmohar, and Mango trees are also commonly planted, Eucalyptus is often preferred for its adaptability and rapid growth.