Complete the below reaction-
$? + {O_2} \to 2{K_2}O$
A.${K_2}$
B.$K$
C.$4K$
D.None of the above

Hint: We have to know that in a chemical equation, the substances which are found in the left side of the arrow are reactants and the ones which are found in the right side of the arrow are products. We can write the general form of chemical equation as,
${\text{Reactants}} \to {\text{Products}}$

Complete answer:
As per the law of conservation of mass, when a chemical reaction happens, the mass of the products ought to be equivalent to the mass of the reactants. Hence, the measure of the molecules in every component doesn't change in the chemical reaction. Subsequently, the chemical equation that shows the chemical reaction should be adjusted. A balanced chemical equation happens when the quantity of the atoms engaged with the reactants side is equivalent to the quantity of atoms in the product side.
We can use the coefficients to make the chemical equation balanced. We can say a coefficient as a number that we use in front of a chemical formula.
In the given equation, we have to identify the coefficient to potassium. We can write the chemical equation as,
$? + {O_2} \to 2{K_2}O$
Here, the oxygen is balanced on both sides. On the reactant side, we need four moles of potassium to make the equation balanced. We can write the completed equation as,
$4K + {O_2} \to 2{K_2}O$

Option (D) is correct.

Note:
We have to remember the two major steps to balance the chemical equation. In the first step, we have to count the atoms which are present in each element of the reactants and products. In the second step, we have to use coefficients and place the coefficients before the compounds as required.