Plant scientists have been able to genetically engineer vegetable seed

Responding to a pm:

Premises:
- Genetically modified vegetable seeds are highly resistant to insect damage.
- They cost more but the cost is likely to decline.
- They need far less pesticide, and most consumers prefer that.

Conclusion: for crops for which these seeds can be developed, their use is likely to become the norm.

A. Whether plant scientists have developed insect-resistant seeds for every crop that is currently grown commercially

Out of scope. The argument clearly discusses only "for crops for which these seeds can be developed". Every crop is not our concern.

B. Whether farmers typically use agricultural pesticides in larger amounts than is necessary to prevent crop damage

This doesn't matter since it will be the same in either case.
GM crops need much less pesticide than do regular crops. Say, GM crops need only 20 kg per acre while regular crops need 100 kg per acre. If farmers do use more pesticides than is necessary, they will use perhaps 110 kg in case of regular crops and 25 kg in case of GM crops. But GM crops will need much less so anyway they will use less than the amount they use in case of regular crops.
If the farmers use only that much pesticide as is necessary, then too they will use much less pesticide in case of GM crops.
So all in all, GM crops are a better deal than regular crops so evaluating this point wouldn't help us judge whether GM crops will be the norm.

C. Whether plants grown from the new genetically engineered seeds can be kept completely free of insect damage

They are much more resistant to insect damage. Whether they will make the crops completely insect-proof, we don't know. It doesn't matter either. Since they are far more resistant, they should become the norm.

D. Whether seeds genetically engineered to produce insect-resistant crops generate significantly lower per acre crop yields than do currently used seeds

The argument discusses the positives of the GM seeds and declares that they will become the norm. But it will be useful to evaluate if they have any shortcomings compared to regular seeds. If their yield is much lower, farmers are likely to not use them since they might earn less than what they earn currently. Hence, this will be useful to evaluate. This will tell us whether GM crops are likely to be the norm.

E. Whether most varieties of crops currently grown commercially have greater natural resistance to insect damage than did similar varieties in the past.

GM crops have much higher resistance as compared with regular crops. Whether the resistance of regular crops has naturally increased over time doesn't matter. GM crops are much more resistant.

Answer (D)