How could a writer best revise the above sentence to avoid overly formal language? cassidy does not know which course she will register for. cassidy does not know for which course she will choose to register. cassidy does not know for what course she will register.

The correct option is “Cassidy does not know which course she will register for.”

This option avoids long grammar structures and makes it easier to read.

The other two options are too wordy and use clauses that do not add meaning to the sentence.  

So the first option is better to avoid the overuse of formal language.

Cassidy does not know which course she will register for.

This is the best way to revise the sentence to avoid overly formal language. It is the one that is the closest to everyday speech. Sometimes when trying to create something less formal it is best to think about how you might hear it said among friends in a lunchroom. It is very rare for a student with his/her friends to say "for which course" or "for what course" unless it is a fragmented question. These are not common phrases found in less formal English.

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