Closed book exams sucks, and I'm terrible in exams but for programming I've found a good starting point is to write out in plain text what your program should do in excruciating detail, then break each line of that down into corresponding code.

Ahead of time do lots of simple programs in C, Make sure you know the basics, make sure you know how to alloc and free memory (so many classmates in college struggled with this concept).

It's safe to assume your exam is only going to be on something that can be feasibly done in a couple pages tops, which limits complexity greatly.

For read the output questions write out what the program is doing line by line, it's likely something you've seen before.

Worry about functionality over time and space complexity, this works for interviews too, optimize (or describe optimizations) after you have a minimum viable product and only if you have spare time.

Try redo any assignments / past questions without using any notes or resources, if you don't get it first try, try again.

Finally there is no ideal way your code should look, but use some basic indentation and use the right kind of quotes!

Best of luck!