Correct answer is (a) Potassium carbonate
To elaborate: Potassium carbonate is a raw material needed to manufacture potash-lime glass and potash-lead glass. Decolorizers are substances which are added to glass in addition to the raw materials in order to avoid any colouring effects due to the presence of iron compounds in the glass. Some of the examples of decolourisers are antimony oxide, manganese dioxide and cobalt oxide.