OG question 824 option D, 

there is a dash. I want to know when a dash can be correct. As in how will it be incorporated in a sentence. Not only for this question but in general, when can I eliminate it. 

Recent interdisciplinary studies advance the argument that emotions, including those deemed personal or private is a social phenomenon, though one inseparable from bodily response.

(A) private is a social phenomenon, though one inseparable

(B) private, are social phenomena that are inseparable

(C) private are a social phenomenon but are not those inseparable

(D) private—are social phenomena but not separable

(E) also as private emotions, are social phenomena not inseparable

A dash can be used to introduce a nonessential modifier. It can be used in place of parentheses or even two commas that separate the nonessential modifier. 

Ex- Thousands of children—like the girl in this photograph—have been left homeless.

If a dash is used to separate the nonessential modifier in the sentence, make sure to use it on either end of the modifier- similar to how you use brackets or commas on either end of the nonessential modifier.  

In the given question, a comma is used to open the nonessential modifier and a dash is used to close it. Incorrect. 

A dash can also be used to show breaks in a sentence.

Just wanted to thank you for a lovely evening  we really enjoyed it.