A substance that changes the rate of a reaction without itself undergoing any net change is A) oxidizing agent B) Reducing agent C) Catalyst D)none?
Answer A is false: an oxidizing agent is a species that loses electrons in a process called (wait for it) oxidation. While this is a very relevant process to chemistry, it has nothing to do with reaction rates as a whole.
Answer B false: a reducing agent is a species that gains electrons in a process called (cue drumroll) reduction. Again, you'll definitely study this process in depth later on, but it is not relevant to the concept of reaction rates.
Answer C is correct: a catalyst will increase the rate of a reaction without being consumed itself. Catalysts play a huge role in many of the biological functions our bodies carry out every day without us even noticing. Enzymes, for example, help speed up a number of reactions that would otherwise take thousands of years to occur (if at all). More on that in this video, if you're interested:
Unless you delve into the depths of biochemistry you will likely not do a great deal of study on exactly how catalysts work. For now, though, your takeaway should just be that a catalyst is a substance that helps speed up a reaction without being consumed itself.
Hope that helped :)