I’m assuming you’re a student.
Your instructor isn’t asking you for the solution. Your instructor is asking you to demonstrate your ability to develop the solution yourself, so that they can give you useful feedback on your ability to develop the solution yourself, because that feedback will help you learn to develop solutions yourself, which is the entire point of the course you’re taking.
Your instructor is trying to help you develop a skill, and the only way to learn to do the thing is to actually do the thing.
It’s a great question, actually. Unfortunately, the answer is probably “because Ken Thompson and Dennis Ritchie liked curly braces.” Or maybe “because parens were already used for function calls and expression grouping, and brackets were already used for array subscripting, so using a different character made parsing easier, but mostly because Ken Thompson and Dennis Ritchie liked curly braces.”
But in any case, that’s the rule: C uses curly braces to delineate blocks of code. And that isn’t a preference; it’s a hard and fast Law of Nature, exactly like “Pascal uses begin and end keywords to delineate blocks of code” and “Python uses indentation to delineate blocks of code” and “English sentences start with capital letters.” and “The gas pedal is under the driver’s right foot.” and “If the ball goes into your goal, the other team gets one point,” and “Royal flush beats four of a kind.”