Answer : The correct option is, (B) 4
Explanation :
Balanced chemical reaction : It is a chemical reaction in which the number of individual atoms of an element in reactant side always be equal to the number of individual atoms of an element in product side.
The unbalanced chemical reaction is,

The given reaction is an unbalanced reaction because in this reaction number of oxygen and hydrogen atoms are not balanced.
In order to balance the chemical reaction, the coefficient 4 is put before the
.
The given balanced reaction will be,

Therefore, the coefficient must be placed in front of
is, 4