The pronoun "this" isn't inherently wrong, but as ChrisLele notes above, its meaning isn't made clear here. We often use "this" in everyday speech in ways that rely on context. The most common example is "What's this?" To understand my question, you'd have to know what I was looking at, holding, or pointing to. On the GMAT, we will generally need additional language to make the meaning clear. Take a look at two examples of "this" used correctly:
Paolo runs at least five miles a day, even when he is tired or ill, and it is this discipline that has earned him the respect of his peers.Here, "this" is clarified as referring to discipline. What discipline? The discipline that he displays by running every day.
We have been told that the facility is safe, but everybody knows this is a lie. What is a lie? There's only one candidate here: the statement that the facility is safe.
By contrast, the use of "this" in A and B is much less clear. Does it refer back to a single noun ("courtship")? Maybe not, but then it needs to stand in for a larger concept, as in the instances above. But is that concept the use of hydrocarbons as perfume, or the fact that this use is important in courtship? The sentence is just too hard to interpret in this form! (In that last sentence, "this form" means "the form we're discussing in A and B."

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