Need help figuring out what the answer is and why it is that answer.

Which of the following is an argument, on its own, for steady state (i.e. zero growth) as societally optimal, eventually necessary for environmental sustainability, or both?

a. The ecological footprint and carbon emissions are currently unsustainable, and must be reduced.

b. Economic activity can only be indefinitely sustained on external energy sources, which are finite, and it is impossible to fully decouple economic growth from energy use.

c.If marginal disutility (i.e. cost) of economic activity increases in scale, approaching infinity, and marginal utility of economic activity decreases in scale, approaching zero, then there must be a finite optimum scale of economic activity.

d. Both b and c are correct.

e. All of the above are correct.