Final answer:
When Company X chooses to utilize the factory space for another product line instead of leasing it to Company Y, it should include the lost lease payments as an opportunity cost.
Step-by-step explanation:
When Company X chooses to utilize the factory space for another product line instead of leasing it to Company Y, it should include the lost lease payments as an opportunity cost. This is because the company is forgoing the potential revenue it could have earned from the lease payments. Including it as an opportunity cost allows the company to account for the financial impact of choosing the product line over leasing the space.