The period of time a structure continues to earn sufficient income to continue operations is referred to as the structure’s:

Option A ('productive use') is incorrect because it's a general term that doesn't specifically address the income-generating capacity of a structure. A property can be in productive use but not necessarily earning sufficient income to cover its expenses.

Option C ('economic life') is incorrect because while related, economic life encompasses more than just income generation. It includes factors like technological obsolescence and market demand changes, not just the period of sufficient income generation.

Option D ('physical life') is incorrect because it refers to the actual durability and physical condition of a structure, not its financial viability. A building may have remaining physical life but no longer be economically viable.