Answer:
In Hawaii (United states)
Explanation:
Mauna Loa observatory is a research facility which continuously measures, monitor and collects the data related to the atmospheric gases since 1958.
The Mauna Loa is one of the volcanoes of the Hawaii island located in the pacific ocean of the US.
The location is ideal for taking the measurement of the carbon dioxide and other atmospheric gases in the form of graphical representations called keeling curve.
Thus, In Hawaii (United States) is the correct answer.
