All inputs can be varied in

The correct answer is: B. Long run

In the long run, all inputs can be varied. This is because firms have enough time to adjust all of their inputs, including capital. In the short run, however, at least one input is fixed. This is because firms do not have enough time to adjust all of their inputs.

Here is a more detailed explanation of each option:

• Short run: In the short run, at least one input is fixed. This is because firms do not have enough time to adjust all of their inputs. For example, a firm may have a factory that it cannot sell or rent in the short run. This means that the firm’s factory is a fixed input in the short run.
• Long run: In the long run, all inputs can be varied. This is because firms have enough time to adjust all of their inputs. For example, a firm that has a factory in the short run could sell or rent the factory in the long run. This means that the firm’s factory is a variable input in the long run.
• Both periods: This option is not correct because it is not possible for all inputs to be varied in both the short run and the long run. In the short run, at least one input is fixed. In the long run, all inputs can be varied.
• None of the period: This option is also not correct because it is not possible for no inputs to be varied in any period. In the short run, at least one input is fixed. In the long run, all inputs can be varied.