The global population milestone of reaching 6 billion was achieved in the late 1990s, and the subsequent milestone of 7 billion was reached in 2011.

 This signifies a significant population growth rate over a span of around 12 years. 

This increase in the world population highlights the ongoing demographic changes and the challenges associated with population growth, resource utilization, urbanization, and sustainability.

THEREFORE THE CORRECT OPTION IS [4].