Which aggregate planning strategy changes output by hiring and laying off workers each month?

The level strategy maintains a steady production rate regardless of fluctuating demand. Instead of adjusting the workforce size, it relies on building inventory during periods of low demand and using those inventories to meet higher demand. This approach does not involve hiring or laying off workers frequently, which makes it an incorrect choice.

A mixed strategy combines elements of both the chase and level strategies. While it may involve some adjustments in hiring or production levels, it does not specifically focus on the regular hiring and laying off of workers each month. Thus, it does not directly answer the question regarding monthly workforce changes.

The outsourcing strategy involves contracting out certain business functions or production processes to external suppliers. This approach does not entail internal workforce adjustments like hiring or laying off workers, which makes it irrelevant to the question.