An independent variable is a.
directly changed by the experimenter.
b.
manipulated by changes to the dependent variable.
C.
a variable that is kept constant.
d.
a variable that is used as a control.

a. directly changed by the experimenter.

Explanation:

In the case of mathematical modeling, analytical modeling , and experimental sciences, the number of dependent variables based upon the amount of independent variables.

The dependent variables represent the result or outcome whose changes is being determined.

An independent variable expressed in the name form like experimental or expected variable, is a variable i.e. created in an experiment in order to verify the effect on a dependent variable

Therefore the option a is correct