Final answer:
The independent variable in a biology experiment involving Daphnia and temperature is B) Temperature. It is what the experimenter changes to see how it affects the Daphnia's heart rate, which is the dependent variable.
Step-by-step explanation:
In a biology experiment involving Daphnia and temperature, the independent variable is B) Temperature. This is because the independent variable is the factor that the experimenter manipulates in order to observe the effect on the dependent variable. In this case, temperature is the condition being changed to see how it affects the Daphnia, specifically looking at their heart rate.
The dependent variable would likely be the heart rate of the Daphnia, as it is what the researchers would measure in response to changes in temperature. An example to clarify these concepts would be that if the temperature of the water in which Daphnia are kept is increased, researchers would then record any changes in the heart rate to determine the effect of temperature on Daphnia physiology.