The first sentence tells us that the terms "sex" and "gender" are often used interchangeably. The rest of the following sentences explain why the term "Sex" and the term "gender" are very different. Thus, it can be inferred that the main point of the argument is to show that even though the two terms are often used interchangeably, this is not correct, i.e. answer choice (D).
Answer choice (A) is in no way discussed in the argument. It is not that the distinctions based on gender are "Frequently arbitrary," rather it is that they are much more complex since gender does not sort people in two nonoverlapping groups. In this way, people can have both "masculine" and "feminine" traits.
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