The correct answer is: None of these.
The percentage of alumina and silica in good fire clay varies depending on the source of the clay. Some fire clays may contain as much as 70% alumina, while others may contain as little as 20%. The percentage of silica in fire clays is also variable, but it is typically in the range of 50-70%.
The ideal fire clay for a particular application will depend on the specific requirements of that application. For example, a fire clay that is used to make bricks for a furnace will need to have a high percentage of alumina in order to withstand the high temperatures of the furnace. On the other hand, a fire clay that is used to make pottery may need to have a higher percentage of silica in order to give the pottery a desired finish.
The percentage of alumina and silica in fire clays can be determined by chemical analysis. This analysis can be used to identify the specific type of fire clay and to determine its suitability for a particular application.