The given figure shows the variation of partial pressure of CO₂ (pCO₂), pH, and temperature (T) over 20 days in a water sample. We need to analyze this data to determine which statement can definitely be made based on the experiment.

1. Analyzing the graph for pCO₂:
   • pCO₂ starts at a lower value and increases over the 20 days.
2. Analyzing the graph for pH:
   • pH starts high and decreases over the 20 days.
3. Analyzing the graph for temperature (T):
   • Temperature starts lower and increases over the 20 days.

Based on the analyzed data, we can conclude the following about the provided options:

• High CO₂ causes global warming: While this statement is scientifically relevant, it is not directly supported or proven by the data from this specific experiment.
• High temperature causes acidification: While increased temperature can lead to acidification, this statement cannot be definitively concluded solely from the given graphs.
• There is a decrease in pH and an increase in both T and pCO₂ over 20 days: This statement directly corresponds with the observed data from the graphs.
• pH and pCO₂ are positively correlated while pH and T are inversely correlated: The graphs show a decrease in pH while both T and pCO₂ increase, which suggests a negative correlation with pH for both factors rather than a positive one with pCO₂.

Therefore, the statement that can definitely be made based on the experiment is:

There is a decrease in pH and an increase in both T and pCO₂ over 20 days.