Which variable is the dependent variable, and why?

A dependent variable is what you measure in the experiment and what's effected during the experiment. The dependent variable responds to the independent variable. It's called dependent variable because it "depends" on the independent variable. In a scientific experiment, you can't have a dependent variable without an independent variable.

Example: You're interested in how stress affects heart rate in humans. Your independent variable would be the stress and the dependent variable would be the heart rate. You can directly manipulate stress levels in your human subjects and measure how those stress levels change heart rate.