A correctly written sentence: A possible first step (in developing) a nonsexist vocabulary with which to analyze the works of the nineteenth-century writer Elizabeth Gaskell would be (to stop) referring to her as “Mrs. Gaskell.”
I know the sentence has no errors, but shouldn’t “in developing” be changed to “to develop” in order to be parallel with “to stop”?
Nope. Can I give you some advice? Instead of saying, “shouldn’t it be X?” when you miss a question like this, say, “Oh, I guess it’s Y. Huh. Now I know!”
Each mistake you make is a learning opportunity. You might be making it harder for yourself to take advantage of that opportunity if you keep thinking you’re going to discover a place where the person who wrote the question is the one who made a mistake.