By which year was the world population expected to reach 6 billion?

The correct answer is that the world population was expected to reach 6 billion by the year 2000. This milestone was notably marked on October 12, 1999, when the United Nations celebrated the occasion. The 6 billion figure was significant as it illustrated rapid population growth trends that had been observed throughout the latter half of the 20th century.

By this time, various factors, including advancements in medicine, agricultural production, and sanitation, had contributed to higher survival rates and lower mortality rates, leading to a significant increase in population numbers. The projections were based on demographic studies, and by the year 2000, the global population was recorded at approximately 6 billion.

The other years mentioned, such as 1990, 2010, and 2020, either underestimated or overestimated this milestone. In 1990, the population was around 5.3 billion, not yet close to the 6 billion mark. By 2010, the population had already surpassed 6 billion, and estimates suggested it would take until around 2025 to reach 7 billion. Thus, the year 2000 is historically recognized as the time when the world population reached 6 billion.