In the year 1999, the world population reached up to:
6 billion.
The correct option is C.
Explanation for the correct option:
Explanation for the incorrect options:
Option A:
The world's population is estimated to have surpassed one billion for the first time in 1804.
Option B:
In 1950, the global population was predicted to be over 3 billion people.
Option D:
The world population was expected to be 7 billion in October 2011.
Final answer: In the year 1999, the world population reached up to 6 billion.