Final answer:
In scientific experiments, variables that can be manipulated are known as independent variables. These can directly influence changes in the dependent variables, which are measured to study effects.
Step-by-step explanation:
The correct option for what is true about variables is b) They can be manipulated. This is because variables are an essential part of any scientific experiment. There are mainly two types of variables in an experimental study: the independent variable and the dependent variable. The independent variable is directly manipulated or controlled by the person conducting the experiment, while the dependent variable is what changes in response to the manipulation of the independent variable. Control variables are also present to ensure that other factors do not influence the outcome of the experiment.