which of the following options is correctly punctuated?

A. the runaway truck which had been parked on a steep hill swerved, and teetered before jumping a curb and slamming into a tree.
B. The runaway truck which had been parked on a steep hill swerved and teetered before jumping a curb and slamming into a tree.
C. the runaway truck, which had been parked on a steep hill, swerved, and teetered before jumping a curb and slamming into a tree.
D. the runaway truck ,which had been parked on a steep hill swerved, and teetered, before jumping a curb and slamming into a tree.

B. The runaway truck which had been parked on a steep hill swerved and teetered before jumping a curb and slamming into a tree.

B is good only if there were some other runaway trucks around. Without commas, you have a restrictive subordinate clause.

To me, we start with
The runaway truck swerved and teetered before jumping a curb.

Now you want to describe the truck, so you say
The runaway truck, which had been parked on a steep hill, swerved and teetered before jumping a curb and slamming into a tree.

Without those commas, you have to ask, "Which runaway truck?"
Answer: The runaway truck which had been parked on a steep hill.

So, I don't like any of the choices: either too many commas, or not enough.

The first two commas in C are correct for the very reasons oobleck gave. This is a perfect example of a nonessential clause, sometimes known as a parenthetical expression.

https://www.englishclub.com/grammar/sentence/parenthetical-expression.htm

I do think, though, that the comma after "swerved" is incorrect -- so that means none of these four choices is right!!

Answer is D

To determine which option is correctly punctuated, let's analyze the different choices:

A. the runaway truck which had been parked on a steep hill swerved, and teetered before jumping a curb and slamming into a tree.
In this option, there should be a comma before "which" to separate the main clause from the relative clause. However, there is no comma before "which," so option A is not correctly punctuated.

B. The runaway truck which had been parked on a steep hill swerved and teetered before jumping a curb and slamming into a tree.
Option B does not have any punctuation between the two verbs "swerved" and "teetered." Since there should be a comma before the coordinating conjunction "and" when joining two independent clauses without a coordinating conjunction, option B is not correctly punctuated either.

C. the runaway truck, which had been parked on a steep hill, swerved, and teetered before jumping a curb and slamming into a tree.
Option C includes commas before and after the relative clause "which had been parked on a steep hill." This is correct because the relative clause provides additional information about the truck. Additionally, there is a comma before "and" to separate the verbs "swerved" and "teetered." Therefore, option C is the correctly punctuated choice.

D. the runaway truck, which had been parked on a steep hill swerved, and teetered, before jumping a curb and slamming into a tree.
Option D has an extra comma after "teetered." Since there is no need for a comma between the verbs "teetered" and "before," option D is not correctly punctuated.

Therefore, the correct punctuated option is C. the runaway truck, which had been parked on a steep hill, swerved, and teetered before jumping a curb and slamming into a tree.