The correct answer is: C. Income from rent of the effective areas used by the production sector.
The decision to manufacture or buy is a complex one that should be made on a case-by-case basis. There are many factors to consider, including the cost of manufacturing, the cost of buying, the quality of the product, and the availability of the product.
The income from rent of the effective areas used by the production sector is not a relevant factor in this decision. This income is not directly related to the cost of manufacturing or buying the product. It is a separate income stream that should be considered when making other decisions, such as whether to rent or buy the production facility.
The other options are all relevant factors in the decision to manufacture or buy. The effective utilization of manufacturing capacity is important because it can affect the cost of manufacturing. The variable cost of products is important because it can affect the profitability of manufacturing. The fixed cost unchanged is important because it can affect the break-even point for manufacturing.
In conclusion, the income from rent of the effective areas used by the production sector is not a relevant factor in the decision to manufacture or buy. The other options are all relevant factors in this decision.