Answer:
Option A, An area of a desert can support a population of 320 cactus plants
Explanation:
The carrying capacity of an ecosystem is the maximum number of organism that can live with in it thereby utilizing its resources and doesn't cause any harm to the ecosystem.
Here in the given options, Option C and D are irrelevant as they do not talk any thing about the ecosystem's carrying capacity.
In Option B, it is said that 1500 bird species currently live in tropical rain forest, but there might be a case where forest has enough resources to allow more birds over and above the current 1500. Thus 1500, is just the present population of bird in the forest and not its carrying capacity
Only option A, talks about the maximum limit of desert to sustain 320 cactus.
Thus option A, is correct as it describes the carrying capacity of an environment i.e desert