The answer is C: When supply is greater than demand.
Explanation for each option:
A. When demand increases while supply stays the same - This situation is not sustainable because the increasing demand will eventually deplete the forest resources if the supply does not increase to meet the demand.
B. When supply and demand are both zero - This scenario is not applicable to sustainability because it implies that there is no forest resource to manage or demand for it, which does not address the concept of sustainability.
C. When supply is greater than demand - This is the most sustainable situation because the forest can regenerate and maintain its resources over time. The supply exceeding demand ensures that the forest is not overexploited, allowing for sustainable management and conservation.
D. When demand is greater than supply - This situation is unsustainable because the forest resources will be depleted faster than they can regenerate, leading to potential deforestation and loss of biodiversity.
In summary, a forest is most likely to be sustainable when the supply of resources is greater than the demand, allowing for regeneration and long-term conservation.