Answer:
Cassidy does not know which course she will register for.
Explanation:
The first option provides a less formal connotation of the statement. First, the sentence structure is simple.
Second, the sentence structure is grammatically correct. If we were to take the last sentence, we would see that:
Cassidy does not know for what course she will register.
For what is unnecessary in this sentence. Therefore, it is automatically incorrect.
Only the first option is correct.
