In 2013, more than three times as many serious automobile accidents we

OFFICIAL EXPLANATION

Note: This OE is for the edited version of the question. While I was splicing two questions, I erred.


This sentence appears short and perhaps even clear-cut.

On the other hand, after you memorize 13,035 idioms and 500 grammar rules (don't do either, please), this question might start to seem tricky.

This trickiness is not helped when the, ahem, transcriber, who was trying to construct a good question for you, spliced two questions incorrectly.
[LARGE SWEAR WORD.]
Thank goodness for JonShukhrat , because I haven't had time to check the boards.

• Which items are involved in this comparison?
Are automobile accidents being compared to drivers?
Are drivers being compared to other drivers?
Or are drivers being compared to the ages of other drivers?

None of the above.

The sentence compares drivers under the age of 20 to drivers over the age of 20.

• How do we know which items are being compared?

→ look at the word(s) to the right of than.
→ we see it
Look at the LHS, before than.
→ On the other side of the comparison only one noun is singular.
→ the singular pronoun it must refer to the age of 20

So we have
In 2013, many more serious automobile accidents were caused by drivers under the age of 20 than over it [the age of 20].

If I were to flesh this sentence out, I would write:
In 2013, many more serious automobile accidents were caused by drivers under the age of 20 than were caused by drivers over the age of 20.

The right hand element of the comparison is the age of 20 ( = it).

Hundreds of threads are dedicated to parallelism and ellipsis or substitution.
As JonShukhrat notes, three official questions [SPOILER ALERT] that are similar to this one can
A similar official question is here, here, and here

Ron Purewal describes a couple of fairly similar question here.

• Split #1: Obvious lack of parallelism: D and E

The RHS of D is than there were caused over it.
→ on the LHS of than in (D) we do not find any there were structure to which this option would be parallel.
→ Nor could we find any; the underlined portion already contains the passive construction, were caused . . . than
The LHS does not use there were as does the RHS. Not parallel.

Option E, than had been caused, lacks verb parallelism.
→ No reason exists to switch tenses to past perfect, the past of the past
→ The sentence is talking about two sets of data from the same year; the two sets of crashes happened during the same time period.
The use of past perfect is not parallel with were caused by

Eliminate D and E

• Split #2 - Perhaps less obvious lack of parallelism: B and C

In the end, we need to be comparing drivers under the age of 20 to drivers over the age of 20

→ Option B fails to make this comparison
(B): In 2013, many more serious automobile accidents were caused by drivers under the age of 20 than caused over it.
→ now the RHS is caused over [the age of 20]
→ the LHS and RHS are not parallel.
it = drivers over the age of 20 = drivers ABC
the pertinent part of the sentence: ... more accidents were caused by drivers under the age of 20 than caused over the age of 20.
I don't even know what "caused over the age of 20" means in the context of this sentence.
The accidents were caused over the drivers?
Option B makes no sense.

(C) looks promising but fails
→ In 2013, many more serious automobile accidents were caused by drivers under the age of 20 than were caused over it
Back to our sentence:
In 2013, many more serious automobile accidents were caused by drivers under the age of 20 than were caused over [the age of 20].
Because we have included the verb but skipped the "by drivers" part, again, it sounds as though the accidents are in an argument with one another about ("over") "the age of 20"?
Takeaway: Make comparisons analysis as simple as possible.
Option A, which seemed a bit odd at the time, maintains the integrity of the logic of the sentence.

The correct answer is A.

Notes

I have corrected the error I created when I spliced two sentences together.
You should never use as . . . than.
(The playing field was somewhat level because every option contains than.

COMMENTS

ShreyKapil08 , welcome to SC Butler.

Wanting to give you all a good question, I took parts of two different questions and put them together

Well handled, everyone.
Kudos to all.
JonShukhrat , thank you for helping—brilliantly, I might add.
Please add a quick analysis or POE at the top of your answer because I have bumped you to Best Community Reply.