Final answer:
The defect causing a system crash after voiding 10 purchases in a row would be classified as Priority Low, Severity High because it's a severe issue affecting functionality but likely occurs infrequently.
Step-by-step explanation:
The proper priority and severity rating for a defect that causes the system to crash only under very specific and less common conditions would typically be considered as Priority Low, Severity High. The reasoning behind this classification is that while the severity of the defect is high due to the fact it causes a system crash (which is a severe impact on functionality), the priority can be considered low because the scenario requiring a person to make and void 10 purchases in a row is not a common use case and likely affects a small number of users. Therefore, the correct answer would be D. Priority low, severity high.