The balance equation is
C2H2 + (5/2)O2 -> 2CO2 + H2O (which is the first option)
Removing the fraction yields
2C2H2 + 5O2 -> 4CO2 + 2H2O (which is the second option)
However the second option is preferred since there is no
fraction.
Having whole number coefficients is sometimes important
especially when the coefficient is interpreted as number of molecules.
