The independent variable is the input and the dependent variable is the output.
The independent variable is seing as the variable that causes the variation of the other variable.
Independent variable is usually represented on the horizontal axis, and dependent variable is usually represented on the vertical axis.
From the graph and your reasoning you can infere that the temperature changes arbitrarily and it determines the flow time.
That means that the temperature is the independent variable and the flow time is the dependent variable.
Therefore, the answer is the option A) only temperature is the independent variable.
