According to a recent study conducted by fire safety experts, on behalf of a major insurance provider, more than three times as many cooking fires occur due to the improper use of gas powered grills
than those that occur due to the incorrect handling of solid-fuel powered grills.
A) than those that occur: Incorrect - as many ...... as is the proper construction. Also the phrase "than those that occur" should have been "than those that occurred"
B) than are occurring: Incorrect - wrong comparison
C) than to occur: incorrect - wrong comparison
D) as those occurring: Correct - this answer choice is grammatically correct and maintains the parallel structure of the sentence.
as many .... as those occurring ...
E) as occur: incorrect - it is missing a verb. The correct form would be “as do occur”.
Answer D