Final answer:
Changing inventory levels is the capacity (supply) modifying option among the given choices, as it directly affects the firm's ability to supply products.
Step-by-step explanation:
The question "Which of the following is a capacity (supply) modifying option?" pertains to actions a firm can take to adjust its capability to meet demand. The correct answer is changing inventory levels. Changing inventory levels directly affects a company's capacity to supply its products. Higher inventory levels mean more goods are available for sale, which increases the firm's ability to meet customer demand. In contrast, methods like changing price or adding advertising or promotion are demand-modifying options as they aim to affect consumer interest and willingness to buy the product rather than the firm's ability to supply it. Changing the product mix to a complementary product could impact both supply and demand but isn't directly a capacity modification. Extending lead times may affect the supply chain but isn't necessarily a way to modify capacity.