Explain Pythagorean Theorem Proof Practice Test

For a triangle with side lengths $a$, $b$, and longest side $c$, which statement correctly matches the inequality to the type of triangle?

A student tries to prove the Pythagorean Theorem by saying: “Assume $a^2+b^2=c^2$. Then the triangle is right, so $a^2+b^2=c^2$.” What is the main problem with this argument?