Any condition in an experiment that can be changed

The factor or condition can be changed is known as a variable. There are independent variables which are variables intentionally changed by the investigator. There are also dependent variables that can be changed as a result of the independent variable. There is also the controlled variable that can't be changed or it would ruin the experiment.
Example:

Scientists are running a test to see if eating junk food makes mice gain more weight than the Mouse Chow.

Independent variable: diet of mice

Dependent variable: weight of mice

Controlled variable: amount of food given

*Note there can be more than one controlled variable
**An independent variable is also known as a manipulated variable
***An dependent variable is also known as a responding variable
Thank You

The most plausible answer is b, duration. the given amount of time is considered how long it last for.