A certain experimental fungicide causes no harm to garden plants, thou

Premises:
- Fungicide causes no harm if diluted 10:1.
- Fungicide eliminates powdery mildew completely from rose plants


Conclusion:
This fungicide, as long as it is sufficiently diluted, provides a means of eliminating powdery mildew from rose plants that involves no risk of harming the plants.
(sounds fair! Let's look at the options for assumptions)

(A) There is not an alternative method, besides application of this fungicide. for eliminating powdery mildew from rose plants without harming the plants.

The argument does not say that there are no other such methods. It just says that this fungicide is such a method. Not correct.

(B) When the fungicide is sufficiently diluted. it does not present any risk of harm to people, animals. or beneficial insects.

Harm to people, animals etc is irrelevant for our argument. We are concluding about safety to the rose plants only.

(C) Powdery mildew is the only fungal infection that affects rose plants.

Other fungal infections are irrelevant to our argument. We are concluding about powdery mildew only.

(D) If a fungicide is to be effective against powdery mildew on rose plants, it must eliminate the powdery mildew completely.

The argument does not talk about "effectiveness". It talks about eliminating powdery mildew in both premises and conclusion.

(E) The effectiveness of the fungicide does not depend on its being more concentrated than one part in ten parts of water.

Correct. It is safe when it is diluted 10:1. It is effective in completely eliminating mildew but it is effective when diluted 10:1? We are assuming that effectiveness does not reduce when diluted so much. Hence this is the answer.

Answer (E)