Premise: You can't prove it true by observation.
Conclusion: You can't prove it true at all.
E. Knowing it's true requires observation.
Negation test: Knowing its true does not require observation.
Think: Oh, if I negate this, I might prove it true with something other than observation, so the conclusion no longer works. TCR.
C. If it's proven by observation, then it can be known to be true.
Think: That's great, but couldn't there still be other ways to know something is true?
You can also try the negation test on C: If it's proven by observation, it still might not be known to be true. After negating, our conclusion (you can't know a math proposition is true) could still work. Cross this one off.
A similar argument might be:
You can't go to the store in Donald Trump's 747 so you can't go to the store.