Which of the following defines the process that decreases the number of slow moving neutrons to prevent the chain reaction in a nuclear reactor from going too fast?

The correction option is D.
Neutron moderation is a  process through which the speed of fast neutrons is reduced in nuclear engineering; it results in turning neutrons to thermal neutrons which are capable of sustaining a nuclear chain reaction. 

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).