In an experiment, the dependent variable is the variable whose behavior you are trying to explain or predict. It is called “dependent” because its value is supposed to depend on something else. That something is the independent variable. When the independent variable changes, the hypothesis is, the dependent variable will change as a result.
For example, let us imagine that we set up an experiment in which we have parents read to their children for varying amounts of time. We want to see if the differences in how much they read to their children will have an impact on the children’s reading levels. In this case, the amount of reading that is done is the independent variable. We expect that, as it changes, it will cause a change in the dependent variable, which is their reading level.
In the example that you have given us, the dependent variable is clear, but the nature of the independent variable is not. The dependent variable is whether people help the stranger pick up things they have dropped. However, we are not told how the circumstances of the dropping varies. The independent variable has something to do with the circumstances of the drop. For example, the experimenters might change the age of the person dropping things or their sex or their physical attractiveness. They might change the number of things dropped. All of these would be independent variables. The researchers would then try to determine if changes in that independent variable caused a change in how often strangers helped pick the items up (the dependent variable).
Thus, in this case, the dependent variable is how often strangers help the researchers pick things up. The independent variable is not clearly stated, but it has to do with the circumstances in which the things are dropped.