Final answer:
A cost driver is a factor that influences the cost of a specific activity. The number of production runs is not a cost driver for selling expenses.
Step-by-step explanation:
A cost driver is a factor that has a direct impact on the cost of a particular activity or process. In the context of selling activities, a cost driver is something that influences the amount of money a company spends on selling expenses. Out of the given options, the number of production runs would not be a cost driver for selling expenses. This is because the number of production runs relates to the production process, not the selling process.