The best advice for the Product Owner in this situation is A "to remind the Product Owner that his primary concern is the flow of value reflected in the ordering of the Product Backlog."
This is because the Product Backlog is an emergent, ordered list of what is needed to improve the product, and it is the single source of work undertaken by the Scrum Team.
https://www.scrum.org/resources/what-is-a-product-backlog
The Product Owner should order the Product Backlog in any way they think will maximize the value of the product, taking into account various factors such as business value, risk, return on investment (ROI), dependencies and impact.
https://www.scrum.org/resources/what-is-a-product-backlog
Therefore, option A is the correct answer.
Option B and C are incorrect, as they suggest that the Product Owner should compromise the value of the product for other factors.