The correct answer is C. Carbon dioxide.
When lime juice is dropped on baking soda, a chemical reaction occurs between the citric acid in the lime juice and the sodium bicarbonate in the baking soda. This reaction
produces carbon dioxide gas, which is what causes the brisk effervescence.The chemical equation for this reaction is:
$$NaHCO_3 + C_6H_8O_7 \to Na^+ + C_6H_5O_7^- + H_2O + CO_2$$
As you can see, the products of this reaction are sodium citrate, water, and carbon dioxide. The sodium citrate is a salt that is
soluble in water, so it dissolves in the lime juice. The water and carbon dioxide are both gases, so they bubble up to the surface of the lime juice.The carbon dioxide gas is what causes the brisk effervescence. It is a colorless, odorless gas that is heavier than air. It is also a greenhouse gas, which means that it traps heat in the atmosphere.
The brisk effervescence is a fun and easy way to demonstrate a chemical reaction. It is also a great way to teach children about acids and bases.