The correct answer is C. Borax.
Borax is a mineral that is used to make glass stronger and more resistant to heat. It does this by forming a network of crystals within the glass that help to prevent it from shattering.
Potassium carbonate is a base that is used to make glass more alkaline. This can make the glass more resistant to acids, but it also makes it more susceptible to thermal shock.
Lead oxide is a heavy metal that is used to make glass more opaque. It also makes the glass more resistant to scratching, but it can be toxic if ingested.
Ferric oxide is a rust-colored mineral that is used to make glass more red. It also makes the glass more resistant to weathering, but it can be brittle.
In conclusion, borax is the best choice for making glass stronger and more resistant to heat.