The term “sex” and “gender” are often used interchangeably. But “sex”

Why not B - it says that gender is more social while sex is more biological?

The argument says that sex and gender are used interchangeably and then explains that there are differences between them.
Hence the correct inference should be that though they are used interchangeably but actually they are not. It highlights some points related to it. Thus what we can strongly infer is that both cannot be used interchangeably. B is out of scope as we cannot infer it for sure.

D as the main point being discussed is regarding the use of sex and gender interchangeably. The other part of passage is giving us the explanation regarding the same.

Posted from my mobile device

(A) Distinctions based on gender are frequently arbitrary.
No its not 'arbitrary'.

(B) Gender traits are not determined at birth.
What is determined at birth is not talked about in the stimulus.

(C) Masculine gender traits are highly correlated with maleness.
No, on the contrary gender traits are NOT overlapping. Its complex.

(D) The terms “sex” and “gender” are not properly interchangeable.
This is the main point of the stimulus. Passage starts off with the author stating that sex and gender are used interchangeably. But then he/she proves its actually not.

(E) Society rather than the individual decides what is considered proper behavior.
This is not talked about in the passage.

Which one of the following statements best expresses a main point of the argument?

When we have to get the main point, we need to Zoom Out.

(A) Distinctions based on gender are frequently arbitrary. -> It is never said arbitrary. And, why do we talk about a part of argument only. Incorrect.

(B) Gender traits are not determined at birth. -> May be or not, but still we missed the "sex" which is also mentioned in argument.

(C) Masculine gender traits are highly correlated with maleness. -> Irrelevant.

(D) The terms “sex” and “gender” are not properly interchangeable. -> It makes sense. The first line is followed by "but", which shows the contrast in the argument and direct of flow. Let's keep it.

(E) Society rather than the individual decides what is considered proper behavior. -> Irrelevant.

So, I think D.

The term “sex” and “gender” are often used interchangeably. But “sex” more properly refers to biological differences of male and female, while “gender” refers to society’s construction of a system that identifies what is masculine and feminine. Unlike the set of characteristics defining biological sex, the set of traits that are associated with gender does not sort people into two non-overlapping groups. The traits characterize people in a complex way, so that a person may have both “masculine” and “feminine” traits.

Which one of the following statements best expresses a main point of the argument?

(A) Distinctions based on gender are frequently arbitrary. - WRONG. Like B and E.

(B) Gender traits are not determined at birth. - WRONG. Like E only.

(C) Masculine gender traits are highly correlated with maleness. - WRONG. Too extreme.

(D) The terms “sex” and “gender” are not properly interchangeable. - CORRECT. True, Words like "unlike" and "while" in the context used suggest that two terms are a bit contra to each other with some overlapping but with a pinch of salt.

(E) Society rather than the individual decides what is considered proper behavior. - WRONG. An overextension may be but not necessarily true.

Answer D.
_________________