In H2CO2, the oxidation number of carbon (C) is +4. This can be determined by assigning the oxidation number of hydrogen (H) as +1 and the oxidation number of oxygen (O) as -2. Let x represent the oxidation number of carbon.

2(+1) + x + 2(-2) = 0

Simplifying the equation:

2 + x - 4 = 0

x + 2 - 4 = 0

x = +4

Therefore, the oxidation number of carbon (C) in H2CO2 is +4.