Answer:
A systematic sample
Explanation:
since there is an increasing trend in the unpaid balances because of the rising cost of housing the best approach is A systematic sample.
A systematic sample is preferred because it will capture the individual debts over the period of 20 years (i.e. the whole 20 years of mortgages ) while a simple random sample will just capture a certain period and its result will not represent the true nature of indebtedness .