subarnozap
What should be the approach to tackle fully underlined SC questions within time frame?
Can anyone help on this?

Identify the Subject-Verb pair and parallel markers (if any) by ignoring the modifiers and prepositional phrases.
This will help you zero-on to the answer, instantly. Initially, it will be difficult, but keep practicing this. I have done the below example for you.
Though now eaten in large quantities around the world and harmless, the tomato is a member of the generally toxic nightshade family, including belladonna, and was once thought to be poisonous itself as a result.A) Though now eaten in large quantities around the world and harmless, the tomato is a member of the generally toxic nightshade family, including belladonna, and was once thought to be poisonous itself as a result.
Striking through the
and is the parallel marker. So, the
participle eaten must have a corresponding parallel entity after
and, but only the noun
harmless appears after
and.
This alone is sufficient for you to eliminate. If you are still doubtful, you can look at the next part of the sentence. It says,
tomato is a member of nightshade family, including belladonna - belladonna is another family like nightshade - this is wrong again in that belladonna belongs to the nightshade family. (In case, you aren't aware that belladonna isn't a family, you can either go further in the sentence or look at other options, and try to understand what they say about belladonna)
Finally,
and was once thought to be poisonous itself as a result. - tomato was once thought poisonous
itselfas a result. The necessity of a
possessive doesn't arise here. Usage of 'as a result' is also very vague.
B) The tomato, though now
eaten in large quantities around the world
and harmless, is a member of the generally toxic nightshade family, which includes belladonna, and
it was therefore once thought to be poisonous
itself.
The participle 'eaten' doesn't have a parallel companion - harmless (a noun), just the way A doesn't. Eliminate.
it doesn't have a clear referent. The necessity of a
possessive doesn't arise here.
Try out on similar terms for C and E. I will tell why D is correct.
C) Once thought to be poisonous itself, the tomato is harmless and now eaten in large quantities around the world, and is a member of the generally toxic nightshade family, including belladonna.
D) Though now
eaten in large quantities
around the world and
known to be harmless, the
tomato was once considered poisonous because it is a member of the generally toxic nightshade family, which includes belladonna.
Participles are parallel and modify the
subject (and the clause has a proper
verb)
nightshade family includes belladonna (properly). This option looks fine (but then, don't arrive at the conclusion, till you have eliminated C and E)
E) A member of the generally toxic nightshade family, including belladonna, the tomato was once considered poisonous even though it is harmless and now eaten in large quantities around the world.
Hope this helps. Let me know if you have any trouble.