Here it says formation of carbon tetrachloride from chloroform is given by the reaction. So here we have one mole of chloroform reacting with one mole of chlorine gas to produce one mole of carbon tetrachloride gas plus one mole of HCl. Now here it says which graph accurately shows the concentration of CCl4 as a function of time for this reaction.
So remember as time progresses, we expect our reactants to decrease in amount and we breakdown these reactants in order to form product. So we expect these to be increasing over time. Now here we're looking at the concentration of CCl4, which we'd expect to be increasing. So A can't work. So the answer is either B, C, or D.
Now remember, your reaction does not go on forever. Eventually all your reactants will be broken down and therefore your product reaches a limit of how much of it can be made. Because of this, B & C don't work because in B it's saying that CCl4 grows exponentially. That can't happen. Eventually you run out of reactants. C is not as steep of an increase, but it is still an increase forever. This doesn't work.
D is the best answer because in D we can see that our product is increasing over time, but eventually it reaches a plateau because we've run out of reactants. Once we've run out of reactants, we can no longer make any more product. So it's going to reach a value or plateau like we see in option D. So out of all the choices, option D is the correct answer.