Final answer:
The challenge for the next century includes making solar energy economical, providing energy from fusion, and managing the nitrogen cycle. These challenges are vital for sustaining the planet's energy needs while mitigating environmental impacts. The correct option is D.
Step-by-step explanation:
The challenge for the next century mentioned in the student's question essentially pertains to energy sustainability and environmental management.
The correct answer to the question, 'What is a challenge for the next century?' is D. All of the above, which includes making solar energy economical, providing energy from fusion, and managing the nitrogen cycle. All these areas represent critical challenges that will likely require innovative solutions in the coming century.
To elaborate, making solar energy economical is essential because solar power represents a super-abundant energy flow that can help us transition away from fossil fuels and fulfill the world's energy demands sustainably.
Advancements in storage technologies and cost reduction are the key factors in making solar energy a widespread solution.
The concept of providing energy from fusion has been in research for many decades. Still, achieving controlled hydrogen fusion on Earth could potentially offer an almost limitless and relatively clean source of energy.
This would profoundly impact the world economy and international relations, currently heavily influenced by oil and natural gas resources.
Lastly, the need to manage the nitrogen cycle is critical due to the adverse environmental effects of excessive nitrogen entering ecosystems, primarily from agricultural activities. Effective management of the nitrogen cycle would involve reducing pollution, moderating use of fertilizers, and sustaining ecosystem health. Option D. is the correct one.