jmc

Isabella wants an average grade of $95$ on her 4 tests; this also means that she wants the sum of her test scores to be at least $95\times 4=380$ (if she goes over this number, she'll be over her goal!). She's already taken two tests, which sum to $97+91=188$, which means she needs $192$ more points to achieve her desired average. In order to minimize the score on the third test, we assume that Isabella will receive all $100$ points on the fourth test. Therefore, the lowest Isabella could have scored on the third test would be $192-100=\boxed{92}$.