The correct answer is A. Grass is not eaten by a lion, however hungry he may be.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. Option B is incorrect because it uses the present progressive tense, which is used to describe actions that are happening at the moment. The sentence is about a general rule, not about an action that is happening at the moment. Option C is incorrect because it uses the passive voice, which is used to focus on the action rather than the agent of the action. The sentence is about the lion, not about the grass. Option D is incorrect because it uses the present progressive tense and the passive voice. This combination of tenses is not used in English.
To explain the sentence in more detail, we can say that lions are carnivores, which means that they eat meat. Grass is a plant, and lions do not eat plants. This is because lions do not have the enzymes necessary to digest plant material. Even if a lion is very hungry, it will not eat grass.