Final answer:
The independent variable in an experiment to investigate how the length of the string affects a pendulum's behavior is the length of the string.
Step-by-step explanation:
In the context of an experiment conducted to investigate how the length of the string affects a pendulum's behavior, the independent variable is the factor that is changed or controlled by the researchers. In this case, the independent variable would be the length of the string. This is because the students are interested in observing how changes to the string length affect the pendulum's period and frequency. The amplitude of the swing, the frequency of the swing, and the number of washers (assuming this refers to the mass of the pendulum bob) would be considered as either dependent variables or controlled variables. According to the principles of physics, the period of a simple pendulum is primarily dependent on the length of the string and the acceleration due to gravity, being nearly independent of other factors like mass and amplitude, especially if the amplitude is less than about 15° as stated in the pendulum lab overview.