Option 1 states that both statements are individually true and statement II is the correct explanation of statement I. Let`s analyze each statement separately:

Statement I: In the Vietnam War, Americans used powerful defoliants such as Agent Orange over large areas of dense forest and agricultural fields. This statement is true. During the Vietnam War, the U.S. military used defoliants, including Agent Orange, to strip the leaves from trees and plants in order to expose enemy troops and disrupt their supply lines.

Statement II: Defoliants are very dangerous war weapons which leave the forests and agricultural fields barren. This statement is also true. Defoliants like Agent Orange are indeed dangerous war weapons that can have long-lasting negative effects on the environment. They can kill plants, contaminate soil and water, and disrupt ecosystems. In the case of Agent Orange, it contained a highly toxic chemical called dioxin, which has been linked to numerous health issues for both humans and wildlife.

Option 1 correctly states that both statements are individually true. Furthermore, statement II does provide a correct explanation for statement I. The use of powerful defoliants such as Agent Orange in the Vietnam War did indeed leave forests and agricultural fields barren.