When enthalpy in a system decreases, the reaction is considered to be
a. neutral
b. exothermic
c. endothermic?

b. Exothermic

Explanation:

Hello,

In this case, it is said that the enthalpy of the system decreases due to the chemical reaction, it means, that the products will have a less amount of enthalpy one the reaction is completed, such decrease will yield a negative change in the reaction enthalpy based on the shown below equation:

Thus, such phenomena accounts for the exothermic chemical reactions which have a change in the reaction enthalpy that is negative.

Regards.

neutrons are in the middle of the atom and the electrons, which are much smaller.