Doesn’t forestall mean to prevent? How can the incipient debates be forestalled if they are already happening？
The word “forestalling” means to prevent or delay something from happening, and “rift” means a serious disagreement between two groups. So, the correct answer is B) Forestalling…rift because the advanced telescopes will provide evidence to either support or disprove the theory, thus preventing or delaying the geologists from engaging in a serious disagreement over the validity of the theory of continental drift.