The ACT English questions that are in the form of "Which of the following alternatives to the underlined portion would not be acceptable?" test you on grammar and usage. There are many different types of these questions, however, and different tactics are helpful for different topics.
To start off, it might be helpful to review the "Grammar and Usage" section of this article about what's actually tested on ACT English and to skim through the 8 common mistakes students make on ACT English. Our article on the best way to review mistakes on the ACT might also come in handy, because it'll help you figure out if you're having issues not only with this question type, but with a specific topic area.
If you really want to get into the nitty gritty of ACT English grammar and usage, I'd recommend going through the PrepScholar guides. I've listed all the topics here, although you might only need to review a few of these concepts, or find that the questions you struggle with fall into one particular category: parts of speech, run-ons and sentence fragments, commas, other punctuation, subject-verb agreement, verb tense and forms, pronoun agreement, pronoun case, idioms, faulty modifiers, parallelism, relative pronouns, and adjectives and adverbs.