How to apply the Principal of Atom Conservation (POAC)
Because in the question it is specifically written
"All of carbon of K2CO3 has been converted ".
Whereas it has not mentioned anything about potassium.
You have to understand that K2CO3 is conberted to K2Zn3(Fe(CN)6)2 by a series of reactions therefore there might be other potassium compounds which had been formed by the series of reactions.
But as it is given all of carbon converted therefore out of the series of reactions there was only one carbon containing compound K2Zn3(Fe(CN)6)2 .
Hope this helps..