What key factor does a common size income statement focus on?

A common size income statement is an analytical tool that presents each line item as a percentage of total revenue. This format allows for an easy comparison of financial performance across different periods, companies of varying sizes, or industry benchmarks. By expressing each income statement item in relation to total revenue, it highlights the proportional contribution of various components, such as cost of goods sold, operating expenses, and net income, helping analysts to assess operational efficiency and profitability at a glance.

This focus on relative percentages enables stakeholders to quickly identify trends and analyze the structure of income, making option C the most relevant answer. Evaluating each item as a percentage of total revenue is particularly useful for benchmarking against peers and assessing how changes within the company affect its profitability over time.