The oxidation number of carbon in carbonate (CO₃²⁻) is +4.

Here's how to determine it:

* Oxygen's usual oxidation number: Oxygen typically has an oxidation number of -2.

* Overall charge of carbonate: The carbonate ion has a charge of -2.

* Setting up the equation: Let "x" represent the oxidation number of carbon. We can write the equation:

x + 3(-2) = -2

* Solving for x:

x - 6 = -2

x = +4

Therefore, the oxidation number of carbon in carbonate is +4.