Explanation

Stimulus Recap

We can’t send spacecraft outside the solar system anytime soon.

The only other possible way to discover extraterrestrial sentience would be via communication.

But communication requires human-level intelligence.

Conclusion: Therefore, we won’t discover extraterrestrial sentience (outside the solar system) unless the aliens are at least as intelligent as us.

Gap in Reasoning

The author assumes there are only two possible ways to discover sentient beings outside the solar system:

1. Sending spacecraft, or

2. Receiving communication.
   
   

If there were some other method , the conclusion wouldn’t follow.

Answer Choices

(A) Wrong. Talks about inside our solar system; irrelevant/out of scope.

(B) Wrong. Adds motivation (“would want to communicate”), but the argument only requires capability, not desire. The "would" is out of scope.

(C) Wrong. Restates the limitations of spacecraft. Doesn’t close the gap.

(D) Correct! Exactly what we need: If beings can’t communicate, the only way to detect them would be via spacecraft. This rules out any third option and locks the reasoning together.

(E) Wrong. Illegal reversal. The argument doesn’t say “if human intelligence, then they will communicate.” It only says “if they communicate, then human intelligence.”