Question 1: what type of language convention has the author used to separate the declarative sentence from the disruptive phrase in this excerpt from "totally like whatever, you know? "

-commas
-question mark
-capitalization
-em dashes

question 2: which word represents the subject of this sentence?

"because contrary to the wisdom of the bumper sticker,
it is not enough these days to simply question authority."

-it
-wisdom
-days
-authority

1. Assuming this is your sentence ("Declarative sentences—so--called because they used to, like, DECLARE things to be true, okay, as opposed to other things are, like, totally, you know, not— have been infected by a totally hip and tragically cool interrogative tone?"), the correct answer is Em dashes. You can see in the sentence that it is incoherent because of that one part which is included between the dashes, and that part disrupts the natural flow of the sentence.
2. The subject of the sentence is the pronoun IT. You can check this by simply asking the question - Who/What is not enough? And you will get the answer - It.