Final answer:
The invalid assumption in CVP analysis is that mixed costs are considered fixed in total within the relevant range. Understanding concepts like average total cost, average variable cost, and marginal cost is crucial to CVP analysis, and mixed costs, being composed of both fixed and variable components, cannot be considered purely fixed.
Step-by-step explanation:
The assumption that is not a valid assumption in Cost-Volume-Profit (CVP) analysis is that mixed costs are considered fixed in total within the relevant range (option d). This is not correct because mixed costs contain both fixed and variable components, and therefore, can't be considered entirely fixed. The key aspects in understanding CVP analysis include concepts of average total cost, average variable cost, and marginal cost. These measures give us information on a per-unit basis and are crucial for making informed business decisions regarding pricing and output levels. By breaking down total costs into fixed and variable costs, firms can better understand their cost structure and how it impacts their overall profitability.
Regarding the assumptions in the question, options (a), (b), and (c) are indeed valid as they align with the principles of CVP analysis. Total costs can be divided into fixed and variable components, the selling price per unit is typically assumed to be constant in CVP analysis, and total variable costs do change in direct proportion to changes in volume.