To make coloured glasses, small quantities of oxides of metals which form coloured silicates are often added to the reaction mixture consisting of Na2CO3 an...

Answer Details

The question is asking which metal oxide is added to the reaction mixture to make colored glasses. The metals added are those which form colored silicates. Potassium, barium, zinc, and copper are all metals, but only copper oxide (CuO) forms colored silicates. Therefore, the correct answer is copper.