As I've said before, the English test is not looking for your ability to find the one correct answer, but oftentimes, is looking for your ability to find the best answer. What that means is that many answer choices may be technically or grammatically correct, but one is superior.
One way the ACT tests this is in your ability to choose the option that is NOT redundant, meaning it does not say the same thing twice. In practice, there are times when being redundant is no big deal, and certainly not something I would count off for on one of my student's essays. But to the ACT, it is an error that must be spotted.
Think about this sentence: "My friend, whose stomach is rumbling, is hungry and wants some food." Though it sounds just fine the way it's written, there's a problem: it's repetitive. If the friend's stomach is rumbling then we know he is hungry, and if we know he is hungry we know he wants some food! There's a much better alternative: "My friend is hungry."
Look at this example from a recent ACT (number 12):
When you read this for the first time, hopefully there is nothing that "sounds" off, and when you read the answer choices, hopefully all of the answers sound similar. That's because they are: all four choices are expressing the same idea, that the note is a thank-you note. But three of the choices are redundant, they say the same thing more than once.
Look at G for example (redundancy in bold): "note of gratitude expressing special gratefulness and thanks"
Now, look at H: "thank-you note on each one expressing special thanks"
Now, look at J: "thankful note expressing special thanks"
Only one choice is not redundant, and that is choice F: NO CHANGE. The word "gratitude" is the only word that expresses the idea that the note is about expressing thanks.
That is why it is better, and thus correct.