The correct answer is option 3, wind breaks. Wind breaks are plantations of trees or bushes that are strategically planted to break the force of the wind. In order to prevent the expansion of deserts, trees are planted in strips or blocks with the shortest trees on the desert side and the tallest on the other side. These plantations act as barriers or breaks for the wind, reducing its speed and preventing erosion and the spread of desertification.

Option 1, shelter belts, is similar to wind breaks and refers to a line of trees or shrubs planted to provide protection from wind, sunlight, or other elements. Option 2, agroforests, is incorrect as it refers to a specific type of agricultural system where trees and crops are grown together. Option 4, social forests, is also incorrect as it is not specific to preventing the expansion of deserts.