landing a human on Mars using solar power to produce electricity replacing human body parts with artificial parts using DNA to identify a criminal
I gave my students this multiple choice question: Which of the following is a current technological challenge, yet to be solved?

Only one answer is correct among these 4 choices:
- landing a human on Mars
- using solar power to produce electricity
- replacing human body parts with artificial parts
- using DNA to identify a criminal

Could you tell me which one is correct?

landing a human on Mars