Answer: D. all of the above
A fallacy of ambiguity occurs when an argument exploits the fact that a word means more than one thing. Here, the word "racing" means more than one thing. On one hand, the word "racing" means "moving quickly." On the other hand, when used to describe hearts, the word "racing" can also mean "rapidly beating." Because the word "racing" means both of these things, the sentence, "my horse is racing fast and my heart is racing" might mean that the horse and the heart are doing the same thing, as the argument assumes. However, it is also possible, and far more likely, that they mean different things: one's horse is moving quickly, and one's heart is beating rapidly. Because the argument ignores the ambiguity in "racing," the argument commits a fallacy of ambiguity.
The argument also commits a conceptual slippery slope fallacy, however. A conceptual slippery slope fallacy is an argument that alleges that a series of actions cannot change the quality of a certain thing. Typically, the problem with a conceptual slippery slope argument is that its second premise claims that something is not a matter of degree, when in fact it is a matter of degree.
In this case, the premise in the argument treats "racing" (in the sense of moving quickly) as an all-or-nothing matter. The argument assumes that either one is racing or is not.
Suppose, however, that racing admits of degrees. Suppose that one could be racing just a little bit, or that one could be racing even more, or one could be racing to an extreme degree. Suppose further that, depending on whether one was walking quickly, somewhat quickly, or slowly, one would be racing to certain degrees. If that is the case, the sentence, "but racing is not significantly different from walking very quickly, which is not significantly different from walking somewhat quickly, which is not significantly different from walking somewhat slowly, which is not significantly different from walking very slowly, which is not significantly different from standing still" is false. It is false because, in fact, there is a difference between walking very quickly, walking somewhat quickly, and so on. That difference, furthermore, is important to racing. The difference is in the degree to which one is racing, though, rather than whether one is racing at all.
Finally, the argument also commits a fairness slippery slope fallacy. A fairness slippery slope fallacy is an argument, which claims that, since a certain course of action is fair, and since any other course of action that differs from the first by a mere incremental difference must also be fair, it follows that a radically different policy must be fair, too.
In this case, the argument claims that, if it is fair to treat racers in exactly the same way as those who are standing still, then this makes it fair to adopt even more radical policies, such as treating racers in exactly the same way as anything.
As we learned this week, however, the problem with most fairness slippery slope arguments, including this one, is that the fairness of more radical policies does not, in fact, follow from the fairness of the course of action in question. In this case, the fairness of treating racers the same as those who stand still would not imply the fairness of treating racers the same as anything.