The correct answer is option 3: Cartagena.

The Biosafety Protocol to the Convention on Biological Diversity (CBD) is an international agreement that aims to ensure the safe handling, transfer, and use of genetically modified organisms (GMOs). It was adopted in 2000 in Cartagena, Colombia.

Option 1: Geneva is not associated with the Biosafety Protocol to the CBD. Geneva is home to many international organizations and conventions, but it is not the location where the Biosafety Protocol was adopted.

Option 2: Nairobi is also not associated with the Biosafety Protocol. While Nairobi is known for its involvement in environmental issues, the Protocol was adopted in Cartagena, not Nairobi.

Option 4: Rio de Janeiro is not associated with the Biosafety Protocol either. While Brazil is a signatory to the Protocol, the adoption of the Protocol did not take place in Rio de Janeiro.

In summary, the correct city associated with the Biosafety Protocol to the CBD is Cartagena, Colombia.