The correct answer is C. Cobalt oxide.
Cobalt oxide is a blue-black powder that is used to color glass and ceramics. It is also used in pigments and in the production of cobalt-based magnets.
Cupric oxide is a red-brown powder that is used to color glass and ceramics. It is also used in pigments and in the production of copper-based alloys.
Nickel oxide is a green powder that is used to color glass and ceramics. It is also used in pigments and in the production of nickel-based alloys.
Iron oxide is a red-brown powder that is used to color glass and ceramics. It is also used in pigments and in the production of iron-based alloys.
Cobalt oxide is the only one of these oxides that imparts a deep blue color to glass. The other oxides impart different colors, as described above.