Concept:Supply can increase from changing plant size only when all factors of production are variable, which is called the long run.
Explanation:In the short run, at least one factor of production, such as plant size or fixed capital, remains fixed.
A firm can raise output in the short run only by using its existing machinery and equipment more intensively, such as by hiring extra labour or using more raw materials.
To achieve an increase in supply by installing new machines, extending buildings, or altering the scale of production, the firm must allow enough time to adjust all inputs.
This time period is called the long run.
In the long run, every factor of production and every cost is variable.
The market period is too short to permit any change in supply.
Normal time is not a distinct period used for supply analysis.
Therefore, a change in plant size is possible only in the long run.
Answer:B. long run.