In 2013, more than three times as many serious automobile accidents were caused by drivers under the age of 20
than over it.
Analysis:
Comparison: Accidents caused by drivers under the age of 20 vs Accidents caused by drivers over the age of 20
Timeline of comparison: In 2013 ( Both events were from 2013. No need to change tense)
Marker: more..than
A) than=> many serious automobile accidents were caused by drivers under the age of 20 than
many serious automobile accidents were caused by drivers over the age of 20.
=> Ellipsis: many serious automobile accidents were caused by drivers under the age of 20 than over the age of 20
=> Concise: many serious automobile accidents were caused by drivers under the age of 20 than over it ( the pronoun 'it' replaces the noun phrase 'the age of 20'
B) than caused=> Incorrect construction: many serious automobile accidents were caused by drivers under the age of 20 than caused over the age of 20 ( we have chance to eliminate the whole common part 'many serious automobile accidents were caused by drivers'. So, why do we need to use caused in this case )
C) than were caused
=> same error as in B
D) than had been caused => Both events happened in 2013. Need to use simple past for both events. No need to use past perfect
E) than there were caused => same error as in B
=> The use of 'there ' is unnecessary here.