Answer:
D. The marginal cost of light is zero, and by convention zero-priced goods and services are excluded from GDP
Explanation:
Only things that have a monetary cost are included in GDP. Things that do not cost "anything" in monetary terms are not included, and this is a major shortcoming of GDP.
From an ecological economics standpoint, things like sunlight, air, and water are often not valued and included in GDP. This is the same case as in the question, because the marginal cost of light is zero, then, it is not included in GDP.