Answer:
(3rd listed choice) Paula's notes are incorrect. Step 4a should be "If the result does not define y as a function of x, then f does not have an inverse," and step 4b should be "If the result defines y as a function of x, then f has an inverse."
Explanation:
If f(y) = x can be solved for a relation for y that is a function, then f(x) has an inverse. It will be the function defined by y.