The percentage of alumina and silica in good fire clay vary respectively is

ANS:A - 25, 75

Fire clay is used to make fire bricks and is generally found under the coal seams. Fire clay contains two major constituents- silica and alumina, of which, the silica percentage varies from 60 to 70% and alumina varies from 25 to 35%. For low duty fireclay 22% to 27% Alumina and 67% to 71% silica.
For intermediate duty fireclay 24% Alumina to 70% Silica.
For high duty fireclay 33% to 40% Alumina and 55% to 63% Silica content.
So, option C gives good bricks than other options. So I think 'C' is the correct option. 20-30% alumina and 55-75% silica.
Ref : SK Duggal Article 2.20.