We are told that the number of tornadoes reported has increased, but the meteorologist hypothesizes that the actual number of tornados has stayed the same, but our ability to detect them has improved. (C) strengthens the argument because if the number of large and medium tornadoes reported has remained the same, the increase in the number of tornadoes reported is attributed solely to small tornadoes. Small tornadoes are arguably harder to find/ detect, thus the increase is logically attributed to the improved ability to find tornadoes as the meteorologist argues.
(E) has no impact on the argument. The fact that the geographical range has remained roughly the same does not mean that the increase in number of tornados reported is due to our improved ability to find them, rather it is equally plausible that the actual number of tornados/ tornado frequency has increased in the same geographical range resulting in more tornados reported.
Let me know if this makes sense and if you have any further questions.