The correct answer is: D. variable inputs
In the short run, a firm in perfect competition can vary its variable inputs, but not its fixed inputs. This is because fixed inputs are already in place and cannot be changed in the short run. Variable inputs, on the other hand, can be changed in the short run to meet changes in demand.
For example, a firm in perfect competition may increase its production in the short run by hiring more workers. However, it cannot increase its production by building a new factory, as this would require a change in its fixed inputs.
The other options are incorrect because they are all factors that can be changed in the long run, not the short run.
- Scale of output can be changed in the long run by building a new factory or expanding an existing one.
- Scope of operations can be changed in the long run by entering new markets or exiting existing markets.
- Structure of technology mix can be changed in the long run by adopting new technologies or abandoning old ones.