1:24, but only because I picked A just after reading first two answers. Not quite safe strategy, but worked this time.
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My explaination goes like this. Please correct me as I got OA wrong.

(E) All of the pigweed seeds that are already on the surface of the soil before the field is plowed will germinate.
Extreme language and irrelavent --eliminate

(D) The pigweed seeds that are churned up to the surface of the soil during the plowing will not germinate unless they are redeposited under the surface of the soil.
This one actually makes sense to me and I marked this one as correct. Reason: in the stimulus, focus on the line "The brief exposure of the seeds to sunlight stimulates receptors". Which means that the seeds have to be redeposited under the soil to make germination work. D perfectly captures this point.

(C) Fewer pigweed plants will grow in the field if it is plowed just before sunrise than if it is plowed just after sunset.
Eliminate : Time is irrelavent here because at whatever time seeds are ploughed, they will eventually get the sunlight as and when the sun rise.
I believe that the point here is that whether they are redeposited after a BRIEF EXPOSURE to sunlight or not.

(B) Fewer pigweed plants will grow in the field if it is not plowed at all than if it is plowed only at night.
Eliminate: If seeds are not plowed AT ALL, how will they be able to capture sunlight? Hence, receptors will not be stimulated and germination will not happen

(A) Fewer pigweed plants will grow in the field if it is plowed only at night than if it is plowed during the day.
Eliminate : Time is irrelavent here. Same reasoning as C. but this one is correct.

Help me out guys.

Thanks
-V

IMO B.
germination, to the best of my knowledge doesn't happen overnight and there is no evidence in the passage that it does. so plowing at night or day does not make a difference. so A and C are out.
E is completely irrelevant.
D. there is no evidence that direct sunlight actually harms the germination process.
Take B. it says if field is not plowed it will have less germination than when it is plowed at night. now take the "at night" part out. the sentence says the field will have germination if it is plowed and no germination if it is not plowed. perfectly right. there is evidence. hence B
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@Vineet : lol .. you are right. that makes no sense..

If you go through the conclusion of the stimulus carefully, you will immediately find the reason why D is the incorrect answer choice.

Lets go through that once.
"Without the prolonged darkness, followed by exposure to sunlight, the seeds do not germinate."

The Conclusion says that without prolonged darkness followed by exposure to sunlight (It is not necessarily Brief), seeds do not germinate. That is not what the conclusion signifies

Choice D incorrectly states that only if there is a brief exposure to sunlight, seeds will be able to germinate.
(D) The pigweed seeds that are churned up to the surface of the soil during the plowing will not germinate unless they are redeposited under the surface of the soil.
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I got your point when you say that "it is not ONLY brief exposure to sunlight that will cause the seeds to germinate" and choice D) is indeed wrong.
However, option A) (the correct answer) is also not helping the cause. Here goes..

(A) Fewer pigweed plants will grow in the field if it is plowed only at night than if it is plowed during the day.

So,if the seeds are burried "all winter" ( prolonged DARKNESS), and then plowed(whether day or night) , they will eventually get the sun light they require to germinate. How can this time factor can make a difference here and that fewer pigweed plants will grow in the field if it is plowed only at night than if it is plowed during the day?

Thanks,
-V