Final answer:
The quickest way to merge markups from multiple copies of a file into one is by using the Merge Markup feature, which consolidates comments, edits, and annotations efficiently.
Step-by-step explanation:
The quickest way to merge markups from multiple copies of the same file into a single document is through the option D) Merge Markup. This feature is often found in document collaboration or PDF editing software. It allows users to combine comments, edits, and annotations from different team members into one master file without having to manually copy and paste each markup. This can greatly streamline the review process in a collaborative work environment. In comparison, options like Copy-Paste could be more time-consuming and prone to errors, while Flatten Document consolidates all layers into a single layer, which could potentially remove the ability to edit individual markups. Import Markup might be a relevant option in some specific software contexts but typically doesn't refer to combining markups from several documents into one. Therefore, the correct answer is D) Merge Markup.