Experiments are commonly performed in order to determine the effect of one variable upon another variable. Those conducting the study will make changes in one of the variables, called the independent variable. They do so in order to determine what effect such changes have on the other variable, known as the dependent variable. 

 

Most physics experiments are about determining the relationship between quantities. In this case, the experimenters wish to determine the relationship between temperature and volume. The purpose could be reworded as "to determine the effect that changing the temperature has upon the volume." This implies that the temperature will be changed by the experimenters. And as a result of those changes, the acceleration will change since the volume values will depend upon the temperature.

As mentioned in the Fundamentals section (above), the independent variable is the variable that the experimenters are changing. The values of the dependent variable depend upon these changes. So re-read the first paragraph and then make your decision about the answers.
​