Answer:
a.using the left pane in the Normal view, because the user can most quickly scroll through thumbnails of the slides
Step-by-step explanation:
using the normal view, it is very easy to reorder the slides quickly and easier.
a slide can not be used to reorder but only to show larger slide preview, therefore the correct option is "a".
a.using the left pane in the Normal view, because the user can most quickly scroll through thumbnails of the slides