Certain genetically modified strains of maize produce a

Let's take a look at the argument first, before examining why (E) is not the answer to this question.

The passage tells us:

• Genetically modified maize produces an insecticide
• This insecticide is present in the maize's pollen
• When the wind spreads this pollen, some of it can land on milkweed leaves that grow near maize fields
• Caterpillars of monarch butterflies only eat milkweed leaves
• These caterpillars die when fed milkweed leaves dusted with pollen containing insecticide

Because of this, farmers are putting monarch butterflies at risk by using genetically modified maize.

Each answer choice suggests a piece of information we could have. The correct answer choice will have the most useful piece of information for evaluating the argument.

Now, let's take a look at (E):
If the caterpillars compete for their food with another insect, they might struggle and fewer of them will survive. However, this doesn't help us evaluate whether the FARMERS are putting the monarch butterfly caterpillars at risk by using genetically modified maize. There is no mention of the maize's pollen that is supposedly harming the monarch butterfly caterpillars.

(E) introduces another risk factor but sheds no light on the argument in the passage -- (E) cannot be our answer.

Here's what (C) says:
The passage said the farmers were putting the caterpillars at risk because the caterpillars were eating the insecticide filled pollen from the genetically modified maize. If the caterpillars are NOT eating the pollen because they do not eat the milkweed leaves when the pollen is being released, then we can't say the farmers are putting the caterpillars at risk.

Knowing whether monarch butterfly caterpillars feed on leaves of the milkweed plant when the maize is releasing pollen would help us evaluate the argument. This is why (C) is the answer to this question.

I hope that helps!