Thanks for the question! While I understand where you are coming from with your point, you have to be careful to directly addressed the issue posed by the question stem. In this case, we are asked to identify the shape of an enzyme activity v. pH curve. Note that we are asked only about a single enzyme. Also note that the pH scale on the x-axis doesn’t include units. This means that we should be thinking about pH in a relative sense and not in a direct sense (in other words, all the way to left of the x-axis doesn’t necessarily refer to pH=0 and all the way to the right doesn’t necessarily refer to pH=14).The way that the question was asked was kind of misleading. It should have asked, "Which of the following is the correct curve that is associated with the common rate of reaction that enzymes operate at?" I chose both B and C because I also remembered that pepsin operates at a very low pH and trypsin, chymotrypsin, carboxypeptidases A and B operate at very high pH..
For this particular question, choice A would represent an enzyme’s activity increasing linearly with pH, which is not typical of enzymatic activity. Choice C represents an enzyme with two activity peaks, which, again, is not typical of enzymatic activity. Indeed, only the parabolic curve corresponds to typical enzymatic activity v. pH graphs.