ANSWER
Temperature
EXPLANATION:
From the question given, we were told that a scientist heated 100mL solution and measured its volume as it became warmer.
To determine the heat of the solution, we need to measure the temperature of that solution. Temperature is the degree of hotness or coldness of a substance. It can also be defined as the degree of internal heat.
So, measuring the temperature of the solution will tell us the amount of heat present in that solution
