Question

Heating of oxalic acid with conc. H2SO4 gives

Answer 1

At first you need to know what happens when the first 2 products are heated, only then you can know what gasses pass trough the NaOH solution.

And that is: The weak and hydrated oxalic acid gets dehydrated& decomposed:

(COOH)2. 2H2O + conc. H2SO4 + heat→ CO(gas) + CO2(gas) + [H2SO4.3H2O]

then these gasses pass trough the NaOH solution:

Co pass-trough since it is not absorbed or reacting with the hydroxide,

but CO2 gets absorbed & reacts & becomes 
Na2CO3