What is the coefficient?
A. A whole number that appears in front of a compound or element in a balanced chemical equation.
B. A whole number that appears as a subscript in front of a formula in a balanced chemical equation
C. A whole number that appears as a subscript at the end of a formula in a balanced chemical equation.
D. A whole number that appears as a subscript at the end of a formula in a balanced chemical equation

A. A whole number that appears in front of a compound or element in a balanced chemical equation.  

Explanation:

That's the definition of a coefficient.

B, C, and D are wrong. Subscripts are not coefficients.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.