Answer: a. More of Project A's cash flows occur in the later years.
Step-by-step explanation:
When a project has its cashflows occurring in later years, the NPV will be less because the discount rate would have a greater period to discount it in as opposed to cashflows that occur more recently which would receive less discounting from the discount rate.
As a result of Project A having more distant cashflows, the discount rate discounted its cash flows more which is why higher rates led to its NPV being zero because those higher rates got to discount it over a longer period.