Here are my thoughts on this. No definite answers but something to think about.

A. Experienced hikers hang back. This option seems self-centered, but there is nothing wrong with looking out for your own safety. Depending on group dynamics it may be the best choice. Just because you have the experience to recognize the situation doesn't automatically make you a formal group leader, and others may not necessarily appreciate the advice - even good advice. So, I admit having done this when the problem didn't seem especially serious or if not in the mood to be "leader-ly".

B. Stay close together. This is a good option, especially if the slope is long but not very steep. This will cause the least delay and may be safe enough for the circumstance, even though other options may be a more cautious choice.

C. Be careful and spread out. This may be the most widely practiced method, but that doesn't make it the best. People sometimes hesitate to call "rock" until they realize it's not stopping. Once a rock is moving downslope and gains momentum it becomes a dangerous and unpredictable threat to everyone below. Even seeing where the rock is headed, it may not be easy or safe to move away quickly.

D. Less-experienced hikers descend first. Why not? But actually telling someone that you want them to go ahead because of the possibility that they might dump a rock on you communicates distrust and could lead to problems working together later. At this point, options A and D seem rather similar.

E. Move one or two at a time. This is the best and safest intervention in a risky spot. If people are already secure and a rock comes loose then there may be no action required, and watching upslope gives the earliest warning possible. If someone is moving safely and not bumping things loose, others can learn from good technique. If someone causes a problem, others can learn from observing poor technique also.

F. All of the above. Circumstances alter cases, and different responses in different situations makes sense. Not always the same degree of risk. Not always the same degree of skill and experience in the group. Not always the same options available.

G. None of the above. Ignoring risk doesn't make it go away.

Falling rock is listed number 2 on Hazards and Risks. Other risks ranked on this list are situations where there is a little more possibility to avoid real trouble. There are times and places in Grand Canyon where risk of rockfall is increased. The scenario given mentions that this is a remote place with no sign of previous travel and this makes a big difference. If there is use by wildlife and/or other hikers, most of the rocks will be stable because the loose ones have already been knocked down. Other considerations are: During heavy rainfall that may trigger a slide. The day after a heavy rain when soils are drying out. As the sun hits the rim in winter and thaws out the overnight freeze.