Final answer:
The independent variable in Lisa's experiment is temperature.
Step-by-step explanation:
The independent variable in Lisa's experiment is temperature. The independent variable is the factor that is being manipulated or changed by the experimenter. In this case, Lisa is investigating the effect of temperature on the heartbeat of a human, so temperature is the variable she is changing to see how it affects the heartbeat. The other options, heartbeat, sunlight, and environment, are not the factors being manipulated in the experiment.