Answer:
Proofreading should occur while the document is being written.
Explanation:
Proofreading is the reading through of a written document for the sole purpose of detecting errors and correcting them. The best time to do this is while it is being written, check through what you have written over and over again to correct any errors made earlier and even give one or two other people to do the same for you because you might unknowingly skip some mistakes that other eyes might notice.