If a country's elderly population is rising rapidly but pension expenditure is not rising at the same pace, which institutional explanation is possible?

Correct answer: Pension coverage may be limited or private pensions may play a larger role

A growing elderly population can increase the potential need for pensions, but the amount paid from the government budget depends on how the pension system is designed. If public pensions cover only some older people, or if employer-based and private pensions provide much of the income, government expenditure may rise more slowly than the elderly population. Eligibility rules and benefit levels can also affect the pattern.

Therefore option A is a possible institutional explanation. Older people do have a relevant connection with pensions, and they do not return to working age. Government spending does not always fall when the elderly share rises; it depends on coverage, financing and policy. The answer appropriately identifies limited public coverage or a larger private role without claiming that ageing has no fiscal effect.