Answer: Only Temperature
Whenever, variables are plotted on a graph, the independent variables are plotted on x-axis and the dependent ones on the y-axis. The graph is formed, when the dependent variable is plotted against the independent one.
Here, in the given graph, temperature is plotted on the x-axis and flow rate on y-axis. Hence, only temperature is the independent variable. Also, flow rate depends on the temperature of the substance and not the other way round.