Answer:
D. Catalyst
Step-by-step explanation:
A catalyst is a substance that speeds up a chemical reaction, but it is not consumed by the reaction. Hence a catalyst can be recovered chemically unchanged at the end of the reaction .
A catalyst lowers the activation energy and hence a large number of reactant particles collide and undergoes reaction. The particles have enough energy since the activation energy which is the minimum energy required for a reaction to occur becomes less.