The para says that a fungus,Ergot, can infect any grains but only common to rye. Ergot is toxic to human. Rye was introduced in Europe in middle ages as a crop for land too poor. So rye is staple for poor.

The question is an inference question so the answer will contain nothing from the outside of the para explained above.

(A) Ergot did not occur in Europe prior to the Middle Ages. - We can't say for sure. Ergot can infect any grain so it might be present prior to middle ages.

(B) People in the Middle Ages were well aware of the toxicity of ergot. - Nowhere in the passage this is mentioned.

(C) Before the introduction of rye, no attempts had been made to cultivate the poor and damp land in Europe. - This is out of scope. No details regarding attempts is discussed in the passage.

(D) In Europe during the Middle Ages, prosperous people were less at risk from ergot poisoning than poor people were. - Here it is. As rye was a staple for the poor, they were at more risk from ergot than rich ones.

(E) Prior to the Middle Ages, Europe was as densely populated as dependence on a single grain crop permitted. - No details related to population is discussed.