Under vertical common-size analysis, each income statement item is expressed as a percentage of revenue. Revenue is calculated as the sum of gross profit and cost of sales (100 + 150 = 250). Therefore, selling, general, and administrative expenses as a percentage of revenue is calculated as:

$\frac{30}{250} = 12\%$

• Option B (20%) is incorrect because it calculates the expenses as a percentage of cost of sales (30 / 150).
• Option C (30%) is incorrect because it calculates the expenses as a percentage of gross profit (30 / 100).