What is the independent variable shown on this graph?
only flow time (s) is the independent variable.
both flow time (s) and temperature (degrees c) are independent variables.
only temperature (degrees c) is the independent variable.
the slope of the trend line is the independent variable.

Explanation:

The independent variable is generally represented on the horizontal axis.

In the graph the horizontal axis is labeled Temperature (°C), so this is the independent variable.

The dependent variable is represented in the vertical axis. In the shown graph it is Flow time (s).

The independent variable is the input of the function or relation and it determines or explains the dependent variable.

So, on the given graph you can you can tell that the flow time (measured in seconds) depends on, or is determined by, the temperature (measured in °C).

The slope of the trend line is the measure of how much the dependent variable changes, on average, when the independent variable changes one unit. For the shown graph, the slope is change in flow time divided by the change in temperature.