In the reaction H2CO3 (aq) + 2NH3 (aq) --> 2NH4+ (aq) + CO3 2- (aq) which substance is reduced?

No substance is reduced, because the reaction is not redox.

Oxidation number of H on the both sides of the equation = +1
Oxidation number of C on the both sides of the equation = +4
Oxidation number of O on the both sides of the equation = -2
Oxidation number of N on the both sides of the equation = -3

There is no change in oxidation number. Therefore, the reaction is not redox.

...... The answer is: E. none of the above