If the amount of atoms of each type on the left and right sides of a reaction differs, what must be done to balance it?
a. coefficients are placed in front of the reactants and/or products
b. subscripts are placed in the chemical formulas
c. it should be left alone
d. superscripts are placed in the chemical formulas

option a. coefficientes are placed in front of the reactants and / or products

For example:

H2 + O2 --> H2O

In the left side there are 2 atoms of oxygen and in the right side there is 1 atom of the same element.

Then you must put a number 2 in front of the product with this result:

H2 + O2 --> 2H2O

Now there in the left side there are 2 atoms of hydrogen while in the right side there are 4 atoms of  the same element. Then you have to put a 2 in front of H2 in the left side, with this result:

2H2 + O2 --> 2H2O

Now the number of hydrogen and the number of oxygen atoms in both sides are equal, i.e. the equation is balanced.