Final answer:
In economics, the short run is the period in which some inputs are considered to be fixed in quantity, such as a lease for a business location. It's not defined by a specific time frame; rather, it refers to the inability to change fixed inputs, while variable inputs can still be adjusted.
Step-by-step explanation:
In economics, the concept of short run and long run are critical to understanding production periods and the flexibility a firm has in adjusting its inputs. The short run is a period where at least some factors of production, such as capital (K), are fixed and cannot be changed. An example of this would be a pizza restaurant that is operating under a fixed lease period and cannot choose a different building to operate in during this term. This implies that in the short run, the restaurant cannot alter this fixed input (the building) even if it wanted to, but it could change variable inputs like labor (L) to adjust its production.
Therefore, the correct definition of the short run in economics is 3) the period in which some inputs are considered to be fixed in quantity. This period is not strictly defined by a specific duration such as less than a year or any other time frame, but rather by the inflexibility to change certain inputs. The flexibility to adjust all factors of production, both variable and fixed inputs, is reserved for the long run, which does not have a fixed duration but is conceptual.