Does it link the balancing to an enthalpy change?
If it wants the equation for the enthalpy change of formation of K2O then 2K + 1/2O2 = K2O is correct as the definition is the formation of 1 mole.
If it wants the equation for the enthalpy of combustion of K then K + 1/4O2 = 1/2K2O is correct as the definition is the combustion of 1 mole. (but asking for the enthalpy of combustion of K is a bit weird)
If it is just general balancing then
2K + 1/2O2 = K2O is best
4 K + O2 = 2 K2O is acceptable