Okay, first, a technical issue - I would really recommend using your highlighting option sparingly. If you highlight half the passage, it doesn't help you at all because you just have a jumble of highlighted material and you still have to go through the passage to find whatever you're looking for.
I think you understand why S(O2) does not change. The passage says that hemoglobin bound to CO is invisible to methods of measuring S(O2). So the patient initially had a S(O2) of 99%. 5% of hemoglobin is then used up to bind CO but of the remaining hemoglobin, 99% still binds O2. Therefore, S(O2) is 99% because the other 5% of hemoglobin molecules are simply invisible.
Actual C(O2) just means actual oxygen content in the blood. Obviously, actual oxygen content went down because you used some hemoglobin to bind CO instead. Note that this "actual C(O2)" does not relate to any measured quantity - it's asking you simply what the actual, objective O2 content is. As they note, if they were to say "measured C(O2)," they would find that B is correct because then C(O2) would have the same error as S(O2). That is, in the equation for C(O2), [Hb] is the same in both cases and S(O2) and P(O2) are also the same because methods of measuring these two quantities cannot take into account hemoglobin bound to CO. So then measured C(O2) would not change.
I think you may be confused about the distinction between actual C(O2) and measured C(O2).
