Chemical equations need to be balanced in order to satisfy the law of:
a. molecular changes
b. conservation of mass
c. conservation of energy
d. products and reactants

The answer to your question is letter B. conservation of mass

Explanation:

A.molecular changes  I think there is no molecular changes law so this option is discarded.

B.conservation of mass  This law states that the number of products is equal to the amount of reactants so if chemical equations are balanced, they satisfied this law.

C.conservation of energy  This law states that the total energy in an isolated system remains constant and is different from the chemical equations so this option is discarded.

D.products and reactants are elements of a chemical reaction but they are not a law, so this option is discarded.