Which of the following is a non-renewable resource
a. gold
b. river
c tree
d. carbon

The answer would be A, Gold.

Once the the amount of gold posited in the earth's crust or mantle is used up or depletes then there is no way to to replenish the gold deposition. This is not the case of the others because you can use reforestation to replenish a forest, water cycle replenishes the river as well as the carbon cycle shows how carbon can be replenished (NOTE: coal is carbon but carbon is not necessarily coal so where as carbon can easily be replenished, it is not the same for coal which is non-renewable).