My vote is for the original sentence.

B & D change the meaning of the sentence.

C is too wordy.

E seems like it could possibly work if you look at it without the modifier "which can be observed from within":

Unlike a hurricane, a tornado is so small as to be impractical of study.

But is that the correct way to analyze it? Not sure. I'm curious to see what others have to say about it.

I believe is E, "such a study" does not refer to any prevous study mentioned. Only a hurricane is mentioned before, not a study. It would be different if the sentence reads: 'a study of a hurricane, etc"

"Impractical of study" is it an idiomatic expression?

OA for this question is A.

i got carried away with D but later realised that a 'tornado' cannot on purpose become so small 'as to' not make such a study possible. i.e. D incorrectly says that the tornado 'did' something with some intent.

1. "so as to" is always wrong, check OG 10.
2. so..that is correct Idiom, which leaves A,B
3. B has pronoun problem- "it", which can refer to tornado or hurricane- hence A.

i think you are confusing 'so as to' with
so [adjective] as to [verb] ( as seen in above sentence.. so small as to )
the above form is valid .

Examples :
The damage caused by the hurricane was so [great] as to [invite] a lot of media attention.

'so as to' as one single phrase is incorrect on the GMAT.

experts pls help.

Cheers,
pd