Prepositions
I try to avoid teaching something by saying "well, that's just the way it is." Hand waving or vague explanations like that doesn't help most people. Yet (here it comes), prepositions are the one thing on the ACT® where that's just that way. I promise this is all going to make sense in a short amount of time.
What is a preposition? For the most part, they are words such as in, on, at, by. These words are used in various ways to convey different meanings. At home, or in home? Those two phrases mean different things. The only way to know the difference between them is to just kind of know them because that's just the way it is.
The ACT® is sometimes going to ask you to look at answer choices and read the context to decide which preposition is correct. Sound easy enough? It can be! Fortunately, native English speakers just know which preposition is correct. As long as you're reading the context and making sure the preposition matches the meaning of the sentence or sentences, you'll be fine.
These sorts of questions can give our non-native speakers some difficulty. My recommendation would be to do your best job to use process of elimination and try to match up the word with meaning in context. These questions are few and far between. But they do show up.