Which tool would you use to measure at what temperature a solution boils?

scale

ruler

thermometer

stopwatch

Personally, I would choose the thermometer for that particular measurement ... especially if I were pressed for time.

The thermometer will deliver the fastest, most precise, most accurate measurement, since scales, rulers, and stopwatches are not designed to measure temperature at all.

Answer: you would use a thermometer

Explanation: why a thermometer? Because a thermometer is the only way a solution can be measured in the context of temperatures. By only way, I mean the hold of the device mentioned in the question