At which temperature would a can of soda be most fizzy when it is opened?

at higher temperatures.
Justification:

1) Soda have CO₂ dissolved. Carbonation consists on that: dissolving CO₂ into water, leading to carbonated water.

2) The solution of a gas into a liquid is inversely related to the temperature: the lower the temperature the more gas gets dissolved.
So, in the manufacturing of soda, the CO₂ is added in cool water in a cool environment.

3) So, the higher the temperature after the soda is delivered, the more gas will be liberated when you open the can.

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