(B) Potato
In the early 19th century, the production of potato brought a fundamental change in the lives of poor people in Europe.
The passage from Chapter 3 mentions that the poor in Europe earlier survived on a monotonous diet of bread and potatoes. This implies potato was the key staple food that had already transformed poor people's diets and lives in the early 19th century. The examiner expects a direct one-word/one-line answer. No explanation is needed for a 1-mark MCQ.