Answer: a. in the short run but not in the long run
Step-by-step explanation:
The Short Run is usually considered in Economics/ Business as a point in time where at least ONE factor of production is FIXED. This factor is usually the Factory because it is hard to change the capacity of a Factory in the Short run. For instance a wing might need to be constructed. Labour on the other hand is considered variable in the Short run though because more people can be hired and the people already hired can put in more overtime.
The Long Run is classified as a point where EVERY factor of production is Variable. There is enough time to even change the capacity of a Factory. So here even Factory is Variable.