The correct answer is B) the use of dense firebricks is not required. The advantage of using boiler furnace studded water wall tubes packed with refractory is that the refractory material, which is a heat-resistant material, can be used instead of dense firebricks. This allows for a more efficient heat transfer and reduces the need for the more expensive and heavy firebrick lining. The refractory material provides the necessary insulation and heat retention, eliminating the requirement for the dense firebricks. The other options are incorrect because A) thinner tubes are not required, C) thicker tubes are not required, and D) lower quality steel is not a direct advantage of using the refractory-packed water wall tubes.